LumoLabs: Shutter-induced blur with an SLR camera

A recent observation made by us and others was that shake reduction efficiency for the Pentax K-7 camera seemed to have a weak spot around about 1/100s and less. Something nobody could really understand and not everybody was able to confirm.

Therefore, we decided to try to answer an old and fundamental question for SLR photography: To which extent does the mechanical focal plane shutter and the mirror slap negatively influence image sharpness? Especially in the digital age with its theoretically rather high image resolution. We, this means two friends (Henning and Rüdiger) and myself (Falk). And of course, we decided to focus our study to the Pentax K-7 SLR camera in order to provde an answer to the observation mentioned above.

The short story is that we managed to find the answers. All our findings are written down in detail in a LumoLabs White paper:

-> http://www.falklumo.com/lumolabs/articles/k7shutter/index.html.

Please refer to this document to actually understand the work we have done. In the following, we will summarize our findings without explaining how we got there. However, note that 4 different camera bodies, data from 4 testers, 8 lenses and two firmware versions have been used. More than thousand test shots and several thousand accurate blur data measurements have been aggregated. High speed video, acoustic recording and acceleration measurements complement the data. So, we assure that the result describe the general behaviour of a Pentax K-7 SLR camera. Pentax has obtained a copy of the paper to be used at their discretion.

We will make no statement about how the results relate to other SLR cameras. Except for a quantitative comparison with one Pentax K20D SLR camera.


Results:

1. The mechanical focal plane shutter indirectly can increase the blur in an image. The exact amount of additional blur depends on the direction in the image. It is zero at a vertical contrast edge (aka yaw blur, blur due to yaw movement). And it is up to 11 µm (on average) at a horizontal contrast edge (aka nick blur, blur due to nick movement).
   
   The exact amount of average blur is shown in the opening figure of this article. It has its maximum for shutter speeds of about 1/100s to 1/80s. It is less than 5 µm for 1/25s and slower. Or 1/250s and faster.
   
   Note that any single image can be affected more or less. Add or subtract +/-50% to get an idea of variation from image to image.
   
   Note that one pixel is 5 µm large and the blur effect is only visible if all other sources of blur are very well under control (sharpening, defocus, shake, subject blur, lens abberation, noise etc.). Normally, these other sources mask the effect. Nevertheless, if you want tack sharp images then you need to understand the shutter blur effect.
   
2. The effect for the Pentax K-7 is larger than for the Pentax K20D. About 2 - 3x larger.
   
3. Mirror slap or shake reduction have no negative or positive impact on the effect. Shake reduction works as advertized but cannot counteract the perturbation from the focal plane shutter as it is too fast really. Mirror slap is very well dampened in the K-7 camera and has no negative impact on image resolution except on a weak tripod.
   
   There is a delay of about 10 ms between end of mirror slap and begin of shutter operation which suffices to keep the mirror slap perturbation out of the image.
   
4. The blur effect is an indirect one:
   
   First, the moving masses of the shutter (curtain etc.) make the body move (with surprising speed and acceleration of its stiff body!).
   
   Second, the body movements cause a classical blur effect lasting as long as the shutter works. The K-7 shutter is faster and stronger than that of the K20D probably increasing the effect by some 60% or so.
   
   Third, the body accelerations cause additional vibrations in the imaging sensor which last a bit longer than the first shutter curtain operates and which magnify the effect by another 60% or so.
   
   Preventing the first from happening (which requires a heavy and sturdy tripod) will kill the effect. There is no "loose" magnetically held imaging sensor and no negative direct impact from shutter curtain or mirror slap causing air flow in the mirror box or whatever.
   
5. In practice, you'll only see any effect with wide angle lenses.
   
   At about 1/100s you would normally have blur due to free-hand shake (we can ignore the case of a tripod as only weak tripods would cause any trouble with the shutter). At 50 mm and longer, the shutter blur will be masked and at 30 mm it will have comparable magnitude. It is at 10-20 mm that the effect will be noticeable most.
   
   In these cases, we highly recommend to shoot at 1/25s (or slower) and to enable shake reduction as it is highly efficient at such exposure speeds. The images will be sharper than at 1/100s!
   
6. Early efficiency tests of the K-7 shake reduction suggested that it may be ineffective at fast shutter speeds as required for long focal lengths. This was a preliminary conclusion we proved to be wrong.
   
   The Pentax shake reduction is effective even at 1/500s! It just cannot prevent the shutter blur at about 1/100s. We may soon publish an update to our SR guide reflecting this.

So, here you have it in a nutshell. Please, refer to the full paper before asking questions. The paper is available as HTML and PDF (linked from the top of the paper). It is recommended to download and read the White Paper on "Understanding Image Sharpness" first.

UPDATE (2010 July, 28):

We checked if the new firmware release 1.10.00.25 released earlier today brought an improvement. The answer is NO.

We've run a number of measurements and within the limits of our very good measurement accuracy (about 0.10 to 0.15 pixels error margins) we cannot see an improvement.

[end of update]

Frequently Asked Questions:

Q: Did you study the K-x, do you know if it has a similar effect?
A: No. But anybody is invited to replicate our study for the Penatx K-x :)

Q: Is the shutter blur in the Pentax K-7 a defect?
A: No, any SLR shutter for any make causes blur to some degree. We just wished for the Pentax K-7 that it would be as small as it is for the K20D. We publish this partly to remind all camera makers that we watch their work ;)

Q: Does switching off shake reduction lead to sharper images?
A: No.

Q: Does mirror lookup work around shutter blur?
A: No.

Q: Does a tripod work around shutter blur?
A: Sometimes. If it is rock solid. A normal tripod most likely won't help much.

Q: Why does a longer exposure time work around shutter blur?
A: Because during the majority of the exposure, the shutter won't move and what you get is an average blur.

Q: Why does a shorter exposure time work around shutter blur?
A: Partly, because there simply is less time for anything to blur. Partly, because stimulated vibrations cause no harm after the shutter already closed.

Q: May I ask questions without reading the White paper?
A: No.

Q: But I don't understand the White paper!
A: How do you know without reading it? ;)

Q: Will you win a Nobel price for this crazy shit of work?
A: No. Alfred Nobel forgot photographers ;)

Further reading:

• Understanding Image Sharpness White Paper
• Understanding Image Sharpness White Paper PDF
• Shutter-induced blur with an SLR camera White Paper (this work)
• Shutter-induced blur with an SLR camera White Paper (this work) PDF
• Why the lens has little effect: Theory of pitch and shift of a camera+lens combination in reaction to moving shutter mass (PDF)

Enjoy the read ;)

Philipp Lahm Wedding

Philipp Lahm is team captain of German national FIFA football team, who just scored third place at the FIFA world championship in South Africa.

First thing he did when he came back to Munich was to marry his bride, Claudia Schattenberg. And as luck would have it he selected the St. Emmeran church of Kleinhelfendorf to do so. Just some of hundred meters away from where my studio is located.

So, I decided to play paparazzo and see what I could grab as a photo.

No question that about hundred true paparazzi were blocking my sight and -- as the number of them exceeded the population of Kleinhelfendorf ;) -- theirs as well. I can only say:

Bring your own ladder!


I am glad I don't have to earn my money this way. As soon as the bride entered the church, they started to run each other over in an attempt to be first to upload the photo. Of course, the 3G/UMTS infrastructure of Kleinhelfendorf wasn't really meant for this use case. To see scores of paparazzi left in frustration is a priceless experience :)

Also, chatting with some of the photographers, TV teams and moderators was interesting. I now have a better idea of the amount of money changing hands on events like this. Needless to say that the right to officially take photos was sold exclusively upfront.


Despite the competition for sight, I tried my luck and here is what I managed to achieve. Enjoy :)


















(Click on the images to enlarge. All photos © 2010 Falk Lumo. No reprint or linking without permission. All and more photos available in high resolution.)

Just a closing remark after you have seen the images... I used my mountain bike to be ahead of the horse drawn wedding carriage on its path thru small and rural roads. After a few kilometers only I've seen no more paparazzi photographers. They don't try as hard as one would imagine... ;)

Pentax shake reduction revisited

Update (April 18, 2010) Breaking News

The entire blog article may only apply to Pentax K-7 with firmware up to 1.00.02.xx with yet to be determined xx. Rüdiger from a German forum has done more measurements with 1.03 which seem to indicate this. I'll keep you updated.

End of Update.


This is a first for me. Because I am going to write about results obtained by others.

Nevertheless, I hope to be able to shed some new light onto an old question: How well does the Pentax shake reduction system work? The result may be surprising which is why I post this article.


1. Information sources

1.1 First and foremost, the admirable work by P. Smith for the Pentax K-7:
- Study of the Effectiveness of Shake Reduction in the Pentax K7
- Discussion of the above original work

1.2 Article by German magazines:
- ColorFoto 7/2008 "8 Bildstabilisatoren von 8 Herstellern"
- Pentax measurement chart contained therein
- ColorFoto 1/2010 "14 Bildstabilisatoren" (only available as print, pp.26-32).

1.3 Own work:
- Quick tests with my K-7
- Re-evaluation of data originally published by P.Smith
- Proposed mathematical model

Let me add that all data I am using (except my own quick tests) are based on a careful examination of edge blur widths (and their variation). Note that edge blur widths can be computed with high subpixel accuracy using the slanted edge method as all sources above do. They compare the shake (motion blur) with static blur caused by the sensor and lens too. Comparisons based on "percentage of useful shots" are not meaningful enough and therefore, haven't been used.

The work of P. Smith uses the Smith shake device aka as his body. The work of ColorFoto uses Steve aka Stabilization Evaluation Equipment which is an apparatus build exclusively for ColorFoto magazine. It was set to a mean shake frequency of 4 Hz and 0.2° amplitude. AFAIK, the shake isn't harmonic which is good.


2. Scope of the work

Aquiring a better understanding of the Pentax SR system. I am not in the boat to examine the 1/100s "SR bug" some report for the K-x and others deny. However, my article may help decide what is a bug and what isn't. My article will also help understand the performance differences of a sensor based system vs. an optical system.


3. A little background

Pentax uses sensor shift-based image stabilization (SR aka shake reduction). It is based on two (or three) angular velocity (gyro) sensors. More about the sensors:
- maRata Gyrostar ENC-03R

The measured angular velocities means that the body knows how the lens pointing direction is shaking and can shift the sensor to compensate. Unlike in-lens systems, it can even compensate for rotations around the axis which are not to be neglected. P. Smith has compiled a number of documents from the Pentax patent application:
- Pentax patents collected by P. Smith

Vendors with sensor-shift based image stabilization include Olympus, Pentax and Sony. Vendors with lens-tilt based image stabilization include Nikon, Canon and Sigma. A few lenses with lens-based stabilization for Pentax exist from Sigma. All systems are actively powered.

It is commonly accepted that neither system is superior to the other. I'll spend a few words on this later. No existing system works in the macro range. Canon has filed a patent requiring additional sensors to address this.


4. A fresh look at existing data


© 2010: measurement data: P. Smith; chart: F. Lumo.

This plot shows the blur width (in pixels) due to shake induced motion blur as a function of exposure time (in milli seconds; e.g., 1/125s = 8ms). The data is taken from the work by P. Smith as cited above. The red curve above is without shake reduction, the green curve is with shake reduction enabled. The thin lines denote upper and lower error margins (based on standard deviation and N=10 sample size). The dashed lines denote a fitted linear line thru the origin.

The camera used (Pentax K-7) has 5 µm pixels and the lens (Sigma 50/2.8 Macro) has 50 mm focal length.


This plot is the same as above with both axes in logarithmic scale. The green dashed line shows a linear line thru zero blur at 44 ms.

It turns out that all data by P. Smith are (within margins of statistical and systematic errors) compatible with the following formula (dotted lines in the above log-log plot):

b = a f |t - t0|

where b be the blur width (e.g., in µm),
a and t0 are constants,
and f be the focal length (e.g., in mm)
and t be the exposure time (e.g., in ms).

and where values are as follows:

SR OFF:
t0 = 0
a = 1 / (280 s)
(of course, a as above is a measurement of P. Smith's body tremor ;) )

SR ON:
t0 = 44 ms
a = 1 / (1200 s)

and b_SRON actually is the minimum of the above formula and b_SROFF. The crossover where b_SRON actually becomes b_SROFF is at t=8ms or 1/125s. For faster shutter speeds, the SR system has no effect (at 50mm focal speed).

The standard deviation of blur width is about the same size as the blur width itself, for both SR on and off.

This corresponds to an advantage of 2.1 stops within a nice range and actually better (~4 stops) around 1/20s - 1/25s. Persons with stronger tremor may see a better reduction.

The nice things about this formula is that we can compute the range of permissable shutter speeds, given a blur width and focal length.


5. Claim

The Pentax formula above holds true for all focal lengths.


6. Backing it up

Wait a second! If true, this claim means that the Pentax SR mechanism isn't able to help aquiring tac-sharp images with long focal lengths! Because below 1/125s, SR basically won't help anymore. It does help aquiring accepable images at maybe 1/50s and 200mm. But not tac-sharp at maybe 1/150s and 200mm. This may then require 1/400s actually, where SR on or off wouldn't matter anyway.

Because this claim is not to be made light-heartedly, I will use more sources to confirm it.

First, my own informal tests involving a visual inspection of images taken with a 300mm lens, at 1/320s, 1/160s, 1/100s, 1/25s, SR ON and OFF: the blur doesn't seem to depend on SR on or off with 1/320s, 1/160s, 1/100s. Blur was less at 1/25s with SR on but still a little bit more than at 1/160s with SR on or off.

Because this quick test isn't academic enough, I consult two additional sources: The ColorFoto tests from 2008 (K20D) and 2010 (K-7). The former measurement chart is online and I try to embedd it here (if it doesn't display, follow the link in the sources section):

© 2008 ColorFoto

We need to look at the second chart here, taken at 130mm and 1/125s. The red bar is with SR off and the dark blue bar to the right is with SR on. As you can see, both bars are roughly of same height, i.e., ColorFoto found SR ineffective at 130mm and 1/125s with the K20D. They actually found blur to be less at 1/15s than at 1/125s...

Now in 2010, I have the paper source for the same test with the K-7 and DA60-250 at 130mm at my disposition. Result: SR on (compared to SR off) has a positive effect of only 10% at 1/200s and maybe 20% at 1/100s. In a range of 1/200s to 1/13s, it remains at about 1 to 1.5 pixels as opposed to 0.5 pixels with tripod. Which is excellent at 1/13s but not so good at 1/200s. Their same curve at 23mm focal length reveals contant, tripod-like blur between 1/30s and 1/8s and even at 1s, only 2px blur. Their result is a little bit less irritating than the earlier K20D result in so far as shorter exposure times didn't lead to more blur.

These are two independent measurements basically coming to the same result: The Pentax SR is designed "to kick in" at exposures longer than about 1/50s.

This leads me to make my claim above.


7. Compared to the competition

In their 2010 study, ColorFoto compared the following SR systems, both at 35mm equivalent and 200mm equivalent focal lengths (FT and APS-C sensors).

35mm: (improvement in stops vs. 1/30s):
Nikon 18-200 VR II: 5
Olympus E3: 5
Panasonic GH1: 3
Sigma 18-50: 3
Pentax K-7: 3 (*)
Canon 18-135: 1
Sony A380: 1
Tamron 17-50: 0

200mm:
Olympus E3: 3
Panasonic GH1: 2
Canon 18-135: 1
Nikon 18-200 VR II: 1
Tamron 18-270: 1
Sony 70-300: 1
Canon 100: 0
Nikon 70-200 VR: 0
Sigma 70-300: 0
Pentax K-7: 0 (*)

(*) I define the number of stops improvement by the time where blur becomes more than 120% compared to a tripod shot, using the 1/focal-s rule to define the 0 stop point. The K-7 had more published shake without SR than the others which can only mean that the higher resolution wasn't corrected for. So, it was ok to add 1 stop to Pentax results (and avoid a -1 stop improvement listing ;) ).

So, all vendors have a common problem already at 100mm (FT) and 130mm (APS-C) focal lengths. At the long end, the best and the worst result are from sensor-shift based systems. At the wide end, they are mixed as well. So, differences are always due to the particular implementation details and not the principle as such.

Looking at results in more detail, I can see the "kick in effect" for the following systems: Tamron 18-270, Nikon 70-200 VR, Canon 100, Sigma 70-300 and Pentax K-7. So, it isn't a system-immanent effect.


8. Pentax SR usage guide

One can compile a usage guide of good exposure times based on the formula given above. This is possible because we can now assume that it holds true for all focal lengths.

The above is a 2D plot of ranges of good combinations of exposure time and focal length. The bright green (tack-sharp) represents 1 µm extra blur due to shake or better (0.2 pixels), the red (blurry) represents 20 µm blur or worse (4 pixels). The two darker shades of green (sharp and soft) represent degrees of blur, which are bearly or clearly noticeable at the 100% crop level.

The border between the two darker green regions represents the standard 135-format 1/f rule (1/(1.5*f) in APS-C land).

The blue or lilac region (blurred) represents a region where blur is obvious but not ruining the shot when looked at from the normal viewing distance: 20µm or 0.02mm is the traditional circle of confusion diameter for depth of field calculations.

One may think that the level of DoF-kind of sharpness be good enough. It depends on the subject. Because a crop from a shorter focal length would have done as well then. Sometimes, the longer focal length would still be the better choice because it collects more light (less noise than the crop) and allows for better focussing.

(Note: the chart and chart description was updated 2010, April 16.)


As can be seen, for focal lengths larger than 100mm, it is getting increasingly difficult or impossible to obtain the required sharpness from the SR mechanism and one has to use the good old rule of thumb. Nevertheless, if one shoots at 200mm and is accepting 1 pixel motion blur, then the avaible range is extended down from about 1/150s to 1/25s or 1/15s even, with the region around 1/100s to be avoided!

One may think that adding a tele lens from Sigma with lens-based stabilization could deliver more headroom for long range tele photo shots. In theory, this may be true. But it remains to be seen if the image stabilization mechanism made by Sigma can deliver for longer focal lengths. It may well be limited to the wide end as well. Additional tests would be required to answer this question.


9. Conclusion

Pentax delivers a capable shake reduction system able to provide up to 4 stops stabilization. However, it is designed to work best at exposure time around 1/20s and therefore, is most useful for normal and wide angle lenses used at low light or in video. Starting at around 100mm focal length, it is increasingly unlikely to see a positive effect from the SR system and beyond 200mm, the SR system cannot be used anymore to produce tac-shap images at lower than usual exposure times.

Olympus shows that this isn't a principle limitation of sensor-shift by delivering best stabilization for longer focal lengths (as far as I am aware of tests). So, there is hope that a future installment of the Pentax SR system is more useful for long focal lengths.

I call it "Tele-SR" and say to Pentax: I want it and I want it now :)


Thanks for stopping by.

Lumolabs: Pentax K-x vs. K-7, sensor and video

Pentax has recently released a new SLR, the Pentax K-x. And while it is positioned at the entry-level market and very competitively priced, it is yet rumoured to have a very good noise performance.

After having done all my lab tests, I may say that the rumors are not true.

The truth is that the K-x may be the 35mm SLR camera which has the best high ISO noise and dynamic range performance to date. This isn't "very good". This is outstanding!

This is my concluding summary which is based on the following individual articles:

• Lumolabs: Pentax K-7 HD video quality
• Lumolabs: Pentax K-x HD video quality
• Lumolabs: Pentax K-7 sensor quality
• Lumolabs: Pentax K-x sensor quality

Noise and Dynamic Range comparison:

At the top of the article is a compilation of various gray sample patches for both cameras, for direct comparison. The patches are from linear raw files (cf. "further reading" at the end of the article).

The resulting 18% gray level chart is like this:


Signal to noise ratio of 18% gray luminosity and color temperature 2900°K (halogen tungsten). Using manufacturer ISO stops 100, 200, 400, 800, 1600, 3200, 6400, 12800. The interpretation of results is the same as for the corresponding graphs on dxomark.com. E.g., print-normalization is for 8 megapixels. The dotted lines indicate the slope for pure photon shot noise.

Obviously, the K-x has no true ISO 100 and both cameras reduce noise even in RAW at ISO 3200 and higher. The ISO sensitivity of the K-x seems to be slightly higher, like at ISO 100, true ISO were more like 87 for the K-7 (DxO's value for the K20D) and 105 for the K-x. However, the above curves have not been left/right-shifted to take this into account.

Gray level noise at ISO 1600/3200 for the K-x is about the same as at ISO 1000 for the K-7. While gray level noise at ISO 1600 and 3200 are about the same for the K-x, there is a minimal loss of resolution when looking at the pattern part in the full test charts. Nothing serious, though.


Signal to noise ratio (SNR) for the Pentax K-7 and K-x. Full measurement for various luminosities and ISO settings at color temperature 2900°K. Measurement similiar to dxomark.com. The dotted lines indicate the slope for pure photon shot noise. The flattening at 30% gray and brighter is believed to be due to systematic measurement errors like imperfect gray patches. The dashed curves for the K-x have been properly normalized to match the higher sensor resolution of the K-7.

The curve for K-7 only and test images are at:
Full SNR Pentax K-7

The curve for K-x only and test images are at:
Full SNR Pentax K-x

The K-x/ISO 200 and K-7/ISO 100 curves are almost identical down to 1% luminosity.

The full SNR graph clearly exhibits the true strength of the K-x sensor: it maintains the good SNR at low luminosity levels. This is an indication that the read-out noise from this sensor is very low.

Therefore, the K-x sensor outperforms the K-7 in low light situations whereas the K-7 outperforms the K-x in good light. This is further confirmed by the better resolution and more artifact-free images from the K-7.


Video:

The quality from HD video is very good. The resolution is as high as the 720p mode allows. It somewhat more moiré and color moiré artifacts than the K-7 and therefore, may not exactly match its analog and film-like quality in 720p mode. But it is very good nevertheless.


Conclusion:

The Pentax K-x has a sensor which rivals the best. That Pentax makes it available in the entry-level class is a good thing. On the other hand, the Pentax K-7 excells in many respects and even its noise performance isn't bad. It is in line with the rest of the competition which is beaten by the K-x in this respect now.


Pentax K-x: "Editor's choice" (entry level, available light)
Pentax K-7: "Editor's choice" (semi professional)



Further reading: Lumolabs testing methodology.

Lumolabs: Pentax K-7 sensor quality

I have previously reported about the image quality from the Pentax K-7.

The corresponding blog articles are here:

• Tungsten high iso noise
• Comparative noise study K-7 vs. K20D
• Comparative resolution study K-7 vs. K20D

Here, I will set a level playing ground for future comparisons of the image quality obtained from various cameras.

In my lumolab testings, I will not report about ergonomics or lack of features etc. The internet is full of this easily obtained information and I will refrain from duplicating it here.


Test chart results:

The photo at the top of the article is an ISO test shot taken at ISO 1600 and color temperature 2900°K. It is a linearly converted RAW file (cf. "further reading" at the end of the article). Go to the gallery (click the image) to have a look at all test shots, available at original full size.


ISO resolution test chart using FA 31 Ltd. at f/4.5. The inner part is 4x. The Nyquist limit is at "7.8" in the inner part. Watch the original at 100% size.

The K-7 resolves down to the Nyquist limit with very little color moiré and false demosaicing. The anti alias filter is rather weak as it allows for moiré down to twice the Nyquist limit. Color moiré is under control though.


Zone plate test chart using FA 31 Ltd. at f/4.5. The big circles have a resolution limit of 1280 LW/PH. The smaller ones are 2x and 4x. The 4x circles at above ~4000 LW/PH have uncolored printing moiré. The Nyquist limit is at 3104 LW/PH which is about 61% into the smallest circle. Watch the original at 100% size.

The transition at the Nyquist limit is rather smooth (the printing moiré being stronger than the sensor moiré) with almost no color moiré.


Numerical lab result:

The numerical evaluation of all noise charts (the full SNR plot) is as follows:

Signal to noise ratio (SNR) for the Pentax K-7. Full measurement for various luminosities and ISO settings at color temperature 2900°K. Measurement similiar to dxomark.com. The dotted lines indicate the slope for pure photon shot noise.

It is possible to derive both dynamic range and noise from the graph. If extrapolating the 0db point for ISO 100 at luminosity 0.07%, then the resulting dynamic range is 10.5 EV (print-normalized to 10.9 EV). E.g., DxO tests the print-normalized dynamic range of the K20D to be 11.05 EV which is about the same. The flattening at 30% gray and brighter is believed to be due to systematic measurement errors like imperfect gray patches.

The theoretical photon noise separation between neighboring ISO lines is 3dB. There are two effects which can be observed:

- The SNR drops faster than required by photon shot noise at gray levels under about 1%. This reduces the dynamic range. Gordon B Good found that this is due to a rather large read-out noise caused by the variable gain amplifiers (VGA) integrated into the CMOS sensor chip.

- The ISO 3200 line is only 1 dB separated from the ISO 1600 line. DxO labs found that this is due to noise redauction applied to RAW data at ISO 3200 and higher. Without such noise reduction, the SNR would obviously be about 2 dB lower.

Overall, the gray level noise (the SNR at luminosity 18.00%) is very close to the competition while the black level noise (the SNR at luminosity 0.10%) is rather large. This is of concern if brightness needs to be pushed in post processing. Interestingly, the ISO 1600 curve provides a better black signal than ISO 100 pushed +4EV.


Conclusion:

- Resolution: Full Nyquist 14.6 Mpixel resolution, weak moiré, very weak color moiré, nearly no demosaicing artifacts.

- Noise: Gray level 40.5 dB at ISO 100, 28.1 dB at ISO 1600, dynamic range (print-normalized) 10.9 EV.

The image quality is excellent at low ISO values and if shadows aren't pushed-processed, good at low ISO values otherwise and in line with the 2008 competition at high ISO values.


Further reading: Lumolabs testing methodology.

Lumolabs: Pentax K-7 HD video quality

I have previously reported about the HD video mode in the Pentax K-7.

The corresponding blog articles are here:

• As a movie camera -- Part I, Part II, Part III

Here, I will set a level playing ground for future comparisons of the video image quality obtained from various cameras.

In my lumolab testings, I will not report about ergonomics or lack of features etc. The internet is full of this easily obtained information and I will refrain from duplicating it here.

The above is a frame from a dumb sample video, available here:
Bavarian Camels (K-7 version).


Test chart results:


Video frame from 720p HD mode. ISO resolution test chart using FA 31 Ltd. at f/4.5. The inner part is 4x. The 720p Nyquist limit is at "8.5" in the outer part. The 1024p Nyquist limit is at "10.0" in the outer part. The color moiré shows that the 720p frames are downsampled from 1024p video frames.

Watch the original at 100% size!


Video frame from 1536x1024p HD mode. The 1024p Nyquist limit is at "10.0" in the outer part and "2.5" in the inner part. The resolved parts with distinct lines is about "8.0" in horizontal and "8.5" in vertical direction, corresponding to 1200x850 pixels in 3:2 or about 1200x717p pixels in 16:9 mode.


Video frame from 720p HD mode. The big circles have a resolution limit of 1080 LW/PH. The smaller ones are 2x and 4x. The Nyquist limit is at 720 LW/PH which is about 67% into the big circle. The Nyquist limit of the 1024p sampling is at 864 LW/PH which is 80% into the big circle. Watch the original at 100% size.
The sampling frequency is clearly visible by the position of the four false color disks positioned at an 80% radius. The false color at and remaining artefacts beyond the sampling frequency is due to a missing video anti alias filter.

A similiar image for the Canon 5DmkII is visible here: Canon 5DmkII zone plate test.


Rolling Shutter Test:


Panning left and right to evaluate the rolling shutter effect.

Note that the sensor of the Pentax K-7 is read out in the opposite direction compared to that of the Pentax K-x, like top-down rather than bottom-up.


Conclusion:

The Pentax K-7 delivers stunning HD video quality, specifically in 720p. It has color moiré artifacts close to the Nyquist frequency which are due to subsampling without an anti alias filter. The artifacts are strong but seem to be nicely confined to a close neighborhood around the sampling frequency. So, in real footage one has to really hit the "wrong" texture to see the effect.

The effect seems to be about that of a Canon 5DmkII along horizontal lines. But the Canon has no such artifacts along vertical lines, pointing to supersampling within lines and line skipping. The Pentax K-7 does line and row skipping. Obviously, the 5DmkII scans 1053 lines whereas the K-7 only scans 1024 lines.

Moreover, in 1024p along nearly vertical edges (cf. the 1024p resolution test chart), one can see the magenta/green fringing and a double seam effect. This is much less visible in 720p though. Additionally, it may be possible to cure it in post processing. The subjective effective resolution in 16:9 is about 1200x717 or 0.9 MPixel. This is a good value actually not achieved by consumer HD camcorders.

Due to its supersampling applied to 720p video, footage from the K-7 looks more cinema-like and analog than footage from entry-level dSLRs.


Further reading: Lumolabs testing methodology.

Pentax K-7 in German magazine test

I don't normally copy/cite content from other sources. But I am going to make an exception now. I cite numeric test results which were recently published by German photo magazine ColorFoto (ColorFoto 9/2009). I do so because there are so few sources around which offer a comparison of the K-7 image quality when brought to lab tests.

You may read more about the K-7 test done by ColorFoto by fetching a printed copy. You may probably be able to download the full text after the printed copy went out of stores, by going here: www.colorfoto.de.

So, besides citing their numeric image quality test results, I am only going to say this much: They liked the K-7 a lot!


Cited test results:

Test (ISO)	Pentax K-7	Pentax K20D	Nikon D300	Canon D50	Olympus E-3
Resolution 100 [LP/PH]	1327	1310	1253	1327	1146
Resolution 400 [LP/PH]	1325	1294	1248	1326	1113
Resolution 800 [LP/PH]	1318	1302	1189	1286	1052
Resolution 1600 [LP/PH]	1295	1287	1144	1191	1053
Texture loss 100	0.0	0.0	0.2	1.1	0.7
Texture loss 400	0.0	0.0	0.2	1.2	0.7
Texture loss 800	0.0	0.1	0.3	1.0	0.9
Texture loss 1600	0.1	0.2	0.3	0.9	1.1
Noise 100 [VN]	0.8	0.8	0.7	0.6	0.9
Noise 400 [VN]	1.5	1.5	0.8	0.9	1.7
Noise 800 [VN]	2.6	2.4	0.9	1.2	2.7
Noise 1600 [VN]	2.7	4.1	1.3	1.5	3.6
Dynamic range 100 [EV]	8.5	8.5	10.5	9.5	8.5
Dynamic range 400 [EV]	7.5	7.5	9.5	9.5	8.0
Dynamic range 800 [EV]	7.0	7.0	10.5	9.5	7.0
Dynamic range 1600 [EV]	6.5	6.0	9.0	9.0	6.5
AF + shutter latency at ~10 EV [ms]	390	280	270	280	360
AF + shutter latency at ~3.5 EV [ms]	580	1060	470	260	730

Notes:
Lenses used (image quality test / AF test):

• Pentax DFA 100 f2.8 Macro / DA 18-55 f3.5-5.6 AL II
• Nikon AF 60 f2.8 Macro / AF-S 27-70 f2.8 G ED
• Canon EF 50 f2.5 Macro / EF 24-70 f2.8 L USM
• Olympus ED Digital 50 f2.0 Macro / ED 12-60 f2.8-4.0 SWD

Light values (EV) in AF tests are computed from quoted 3000/30 lx figures by myself.

The image tests have been run from the JPGs produced out of camera. The default setting of the K-7 (as used in the test) applies noise reduction starting at over ISO 800 only which explains the relatively good values at ISO 1600. Most other brands obviously already apply noise reduction at ISO 800 or below.

The ColorFoto lab tests may be unique in that they provide a measurement of the loss of texture (typically caused by aggressive noise reduction algorithms).

Therefore, one may have a careful look at the "texture loss" measurements. Pentax has none, Nikon has it somewhat under control and Canon is more like a mess. So, depending on how you weight noise vs. texture, either Nikon or Pentax would win in the noise/texture department. Dynamic range when measured from JPGs does always include the tone curve applied. As there are plenty of tone curve parameters to play with, esp. on the K-7, I don't know how useful the corresponding measure is. I included it for the sake of completeness of citation.

More interestingly though, the K-7 sensor seems to have no significant difference to the K20D sensor (to be confirmed by raw file lab tests though) and the K-7 JPG engine seems to offer subtle improvement in resolution and noise/texture. This is in line with what I found out myself (cf. my other blog articles). And, this test sharply contradicts another blog article ricing high concerns...

Another figure being noteworthy of course is the AF measurement. In low light, the K20D AF.S figures were last in the field. The K-7 now seems to be more comparable to Nikon or Olympus with Canon still winning the competition. But the results are to be taken with a grain of salt: The difference of figures in full light between K-7 and K20D showcase errors in the measurements.


Conclusion

The first lab test of the K-7 image quality I am aware of confirms my early impression: image quality is on par with K20D or better. The JPG engine still produces a lot of noise but preserves a lot of texture detail too, compared to its direct competition.

LumoLabs: Busting the alignment myth

This will be a special feature for hard core falconeye followers ;)

It all started with the exciting new feature of the Pentax K-7 camera which can produce true HDR images in-camera (it takes 3 shots, 3 exposure values apart from each other, and creates a single, tone-mapped or blended image from the sequence). It does it so well, esp. in the medium setting (as opposed to strong) that a limitation of the feature does actually hurt: you require a tripod to use the function.

The general opinion why Pentax left out the alignment step from their HDR function is that it is a processing power-hungry computation not easily done in-camera.

Not my opinion, though. I felt that Pentax has left out 10% of the implementation which reduces 90% of the functionality. Shouldn't it be the other way round?

Now, opinion isn't proof.

So, I implemented my own alignment operator, benchmarked it and compared its quality against the state-of-the-art in HDR.


The algorithm

The implementation is based on an algorithm whose computing time increases linearly with image size and is constant with amount of distortion. Note that I did not attempt to cope with the alignment problem known in panorama stitching, i.e., I don't do SIFT key extraction, I don't correct for lens distortion, I don't project onto a sphere before alignment, etc. pp. However, what I do is correct for shift and rotations which is a mathematical first order approximation to the latter and rather exact for small shifts and rotations.

The implementation uses a monochrome quad tree and high bit depth. The coding is in Java. It isn't based on anybody else's intellectual property.


The benchmark

I benchmarked the algorithm with a 5 image HDR from Munich Siegestor. It had distortions of up to 20 pixels and needed rotation as well. The source images haven't been ideally sharp (e.g., 1/25s and wide open with 15mm). And no program created a perfect alignment. So, I considered this example be adequate. The images were 14.6 MPixel JPG images.

The processing times on a Mac Mini (Early 2009), one processor used, are as follows:

• Single alignment: 175 ms
• Alignment of all HDR images: 700 ms
• Reading of all images from disk into Java heap: 16 s
• Shift-turn and write images to disk: 34 s
• Total, disk to disk (70 MB transferred), 5 images: 50 s

The K-7 has a dedicated image DSP and can certainly rival this. E.g., it can JPG-compress a 14.6 MPixel image in about 100 ms only!

Because the HDR function within K-7 already has created the required image structures in its 2 GB memory, it would be conclusive to assume that addition of an auto-alignment feature to the HDR function would add less than an extra 1/2 second to overall operation time.

If processing speed isn't the reason, so maybe it is quality? This must be looked at. In particular as some of the alignment operators I compared with took several to nearly 10 minutes to complete.

The quality

I've run several alignment operators and come up with the following ranking:

1. Photoshop CS3; PhotoAcute ("normal alignment")
2. Falk Lumo Operator (as of this blog article); PhotoMatix 3 ("by matching features")
3. PhotoMatix 3 ("by matching features"; "reduce ghosting")
4. PhotoMatix 3 ("by correcting horizontal and vertical shifts")
5. No alignment

All pre-aligned images have been read into Adobe Photoshop CS3, either by HDR merge (without further alignment) or by opening the 32 Bit .hdr-file written by PhotoMatix. Tonemapping was done in Photoshop CS3, using the "Equalize Histogram" method. The latter isn't best but as it has no parameters, I thought it would make for a good option in a comparative study. Finally, the result was studied to derive the above ranking. Which is entirely subjective!

Below are the two samples from category #2. Please, visit the gallery to view the other samples, too. Note that all sample images have been resized to 6 MPixel.

Fig.1 No alignment


Fig.2 PhotoMatix


Fig.3 Falk Lumo Alignment Operator (as decribed here)

Take me to the gallery

Conclusion

The myth is busted. There is no reason to not include auto-alignment into the next firmware release of the K-7 HDR feature. In less than an extra second of processing time, about PhotoMatix quality can be provided. Make it so!

Pentax K-7 Concluding Review Report

Fig. K-7 getting wet feet while recording a movie (aperture blades stopped down).


This is my concluding review report about the Pentax K-7 dSLR camera hitting stores next week (last week in Japan).

It is based on my experience throughout the past month, after Pentax Germany was so kind to ask me to test their new flagship camera. It is today that I have to return the camera :(

You may read about my detailed findings in my various blog articles found here:

• I am Pentax K-7 Alpha Tester :)
• In the rain
• Its continous autofocus -- Part I, Part II
• Its precision autofocus
• Tungsten high iso noise
• Comparative noise study K-7 vs. K20D
• Comparative resolution study K-7 vs. K20D
• As a movie camera -- Part I, Part II, Part III
• Ergonomics

Again, I am not going to repeat the spec sheet of the camera. Please, go to

• pentaximaging.com K-7 website
• K-7 user manual
• DPReview web preview/review

to get the full list of features. Here, I'll just share my opinion about the product. First, I am going to break up my impression with various "use cases" in mind.

1. General comments

The Pentax K-7 is in a class with only five competitors (*). It is Nikon D5000, D90, D300, Canon EOS 50D, and Pentax K-7. The price range covers 730$ (D5000), 950$ (D90), 1200$ (50D), 1300$ (K-7), 1700$ (D300). However, as the K-7 is at launch price now, it is expected that the street price of the K-7 is to drop below 1000$ later this year and would be priced roughly like the D90.

So, let me quickly compare the K-7 against the other four models:

• K-7 vs. D5000
  Pro: Better LCD, Faster/better shutter, much better VF, SR, WR, metal body, 15MP
  Con: Heavier, no tilt LCD


• K-7 vs. D90
  Pro: Faster/better shutter, slightly better VF, SR, WR, full metal body, 15MP
  Con: none


• K-7 vs. 50D
  Pro: Slightly better VF, video, SR, WR
  Con: 1/180X, no double cross center AF sensor, no tethering


• K-7 vs. D300
  Pro: Lighter, video, SR, 15MP
  Con: slower and no 150k shutter, 1/180X, no 51 AF points (11), no 1005 zone metering (77), no tethering

_____
(*) defined by dSLR, 12+ MPixel APS-C, LiveView, ISO3200+, 3.5+ fps
SR: in-body stabilization (shake reduction)
WR: sealed body and kit lenses for weather resistance

This little list is not meant to rank the competition ;) But it shows that the K-7 is very interestingly positioned. Let's say that the D5000 seems to be an interesting camera but is positioned below the D90 which is a more serious competitor, but still somewhat below the class of the K-7. The only true matches in class are 50D and D300, where D300 actually is somewhat above. However, if video or in-body stabilization or bad weather-resistance are required then there simply is no match for the K-7. The only thing we need to check out is whether K-7 delivers on its promise. If it does then it has no real competition (yet).

Having said this, I'll leave the feature list comparison for a more meaningful consideration of strong or weak fields of application. Each of the following fields gets a score within 1 to 5 stars (category for digital 35mm mount SLR).

Throughout, this is based on the following judgements:
- High ISO noise performance is on par with the current state-of-the-art for APS-C (ignoring differences of a 1/3 stop). The K-7 will look both more noisy and more detailed at the pixel level compared to the competition above. Some think that this renders more beautiful images when printed. Others think that buttery images at high ISO are better. I think that the differences aren't significant enough to be important but I prefer to keep noise and detail for later processing. Full frame delivers one full additional stop though.
- Autofocus is significantly improved compared to previous models from Pentax and at or above average. But still no match for the best in its class (D300).
- 15 MP resolution is fully applicable when using good glass.
- Everything what you can read in the separate articles as given above.


2. Landscape / Cityscape photography
★★★★★
Actually, the K-7 is a stellar performer here and K20D already scores high. The relatively lightweight body, stellar image quality at low ISO in combination with available stunning prime lenses makes this the best choice short of a (heavier) full frame 20+ MP camera. A function for automatic horizon just makes it perfect.

3. Wildlife
★★★☆☆
It scores high like for landscape, but I remove two stars for the following reasons:
- fast and/or long glass is extremely rare and expensive, and a tele converter is lacking.
- AF.C autofocus is good (and much better than in previous Pentax cameras) but may still miss a shot in action-loaden situations (flying birds, predator attack).
Note that the first point is not a weakness of the camera itself.

4. Sports and Action
★★★☆☆
Same rationale as for wildlife. Also, there are faster, machine gun type bodies.

5. Wedding
★★★★☆
The K-7 is the perfect APS-C body for this kind of shootings and additionally can provide stunning HD footage. It is a robust tool and sufficiently fast, too. However, in situations where light is not optimal, a full frame body with fast glass can be a better option.

6. Family
★★★★☆
Much like wedding. But ease of use and the fact that full frame would be overkill adds a star. Very young active kids or pets can still be challenging though and costs a star.

Btw, I don't agree with some other reviewers that the K-7 underexposes. If I would, I would subtract another star here. I agree that K-7 tends to expose darker than what is seen in other cameras, but for a reason. Easily studied with its live histogram. E.g., a burned highlight in the extreme quarter decreases exposure from 1/25s to 1/30s. So, it isn't true that K-7 underexposes to rescue irrelevant highlights. It just takes into account that bright levels clip where dark levels don't. Just work with +1/3EV if you never adjust levels.

7. Street and People Photography
★★★★★
Can't be beaten. Light-weight un-obstrusive and able to cope with almost every foreseeable situation. A smaller camera (like K-m or non SLR) may be interesting but would be less rugged. The improved metering performance with highlights and with flash are important improvements coming with the K-7 (for night life).

8. Travel
★★★★★
Same rationale as above. Just keep an eye on your stuff, though. An SLR may not always be the best option. Depending on your priorities.

9. Portrait and Studio
★★★★☆
The K-7 is almost perfect. The lack of a tethering option costs a star, though.

10. Movies
★★★☆☆
I judge the K-7 to be the second most interesting SLR offering video (with Canon 5DmkII being no.1 which would get 4 stars). The missing 2 stars are:
- No supersampling of full 15MP down to 1080p
- No videographer form factor/controls/EVF available with working Contrast AF
(taking the strengths (ISO, DoF etc.) into account.)

What sets it apart from other offerings are a "better than 720p" mode @30fps, uncropped 3:2 video, stereo, bearable jello effect, some manual control (aperture, EV compensation, exposure lock), no motion-compression artifacts. Of course, full manual control, electronic viewfinder and a fast contrast AF are all missing.

11. HDR and panoramas
★★★★★
The built-in HDR feature makes any other camera score 4 stars or less only. But Pentax missed the chance to align the images in-camera which would have been straightforward to do. The feature works great, otherwise. Moreover, the K-7 offers perfect control to do HDR panoramas. Live view with grid, exposure bracket and horizon control are an additional help.

12. Macro photography
★★★★☆
Perfect control available. A higher resolution LV would be an improvement but doesn't currently exist on the market. I subtract a star because an automatic focus series for focus stacking would be great. AFAIK, it doesn't currently exist on the market for no reason.

13. Astro photography
★★★☆☆
LiveView is of great help and controls are perfect. Missing though:
- Tethering option.
- Dark frame subtraction shut off for exposures > 30s not possible.
- Crop of FoV compared to full frame, depending on the type (FoV) of telescope.

14. Final Verdict
Four stars is the average and my conclusive score. If you look at my criteria, there probably will be no camera scoring at 5 stars (smarter beats bigger). Certainly not its direct competitors, the 50D and D300. I like both cameras as well and they have different weaknesses and strenghts. But I like the K-7 better. A contender for 5 stars would be Nikon's D3X, if it were 1/3 lighter and had video.

So, the K-7 not only feels like a lot of fun to use, it is a very good proposition on the current market. The best ever made by Pentax and a bigger step forward for Pentax than they did whith the (already very good) K20D. Compared to the K-7, the K20D feels old already.

So it does deliver on its promise indeed which means:

Pentax K-7

★★★★☆

Editor's Choice

"APS-C SLR camera"

(5 stars not assigned in category; as of 2009, July 1st)

A sunny sunday

Enough testing. The Alpha Test is going to end anyway soon. Just relax, sit outside in the garden and have a good time. Which doesn't mean that I cannot press the shutter once in a while ;)


And then I saw a red balloon in the distance.

Shot freehand with a Pentax K-7 and a FA* 300/4.5 lens. You may click on the image above to view it in original size and see that the opening image actually is nothing but a 100% crop from this very image. You actually can see heads more than 3 km or 2 miles away (as I later computed).

Enjoy ;)

K-7 and ergonomics

This is just a short essay about 5 ergonomic details in the K-7 which may create worries.


1. The omission of the Shake Reduction button

The K20D has a clearly visible SR button which is missing in the K-7. The following sequence is required in the K-7 to switch SR on or off. First, lets define how I call the buttons around the OK button: "8", "4", "5", "6", "2", like on a numeric keypad, with "5" being the OK-button itself ("8" is the drive button on top of "5").

[INFO] -> [2]/[6]{0-6} -> [RearDialClick]

{0-6} means that the button must be pressed 0 to 6 times, depending on where it was before. Assuming you only switch SR, it is just two buttons, [INFO] -> [RearDialClick]

Because SR is switched off automatically, e.g., on a tripod with remote control, this omission is no big deal.


2. The omission of the exposure bracket button

The K20D has an exposure bracket button which serves to quickly enable exposure bracket using the two dials to set #images and EV distance in between.

Assuming you always use the same exposure bracket (e.g., 5 exposures 2EV apart), the enabling sequence is as follows:

enable: [8] -> [4] -> [4] -> [5]
disable: [8] -> [6] -> [6] -> [5]

So, it is 4 button presses to activate and de-activate. If used often, one can live withit. But it is a lot less fun to set than with the K20D. Again, this omission is no deal breaker though.


3. Pressing OK for AF selection

Not being a heavy user of AF point selection, I can live with pressing OK prior to AF point selection. A user which relies on AF point selection however, will not understand why the logics isn't inverted as soon as you dial into AF point "SEL" mode. Because now, a change of AF point is much more frequent than, e.g., a change of drive mode.

So I really hope that Pentax adds a custom setting which makes AF selection the default in "SEL" AF point mode where pressing OK acts like Fn.

-- UPDATE --
As of today (July 29, 2009), Pentax has released a firmware update (v.1.01) which adds exactly the behaviour as described above as an option. Now, AF point selection can be used like with a K20D (i.e., the OK button will act as mode switch, not as middle AF point selector).

Pentax, thank you for listening!
-- END of UPDATE --


4. The mode dial lock button (mdlb)

The mode dial of the K-7 on the left side controls mode such as P, Av etc. In order to prevent that it can be changed inadvertently, it got a button in the middle to lock it. It must be pressed in order to turn the dial.

I developped a technique using three fingers from my left hand: the index finger presses the lock while the two neighboring fingers (one is the thumb) turn. Any other technique including using two hands (the obvious choice) turned out to create headaches.

There are two problems with the mdlb:

• Engaging it will more probably lead to an inadverted change of the exposure meter mode (the level right underneath!) than the mode would change w/o the mdlb. And this inadverted change passes unnotices most of the time!
  Therefore, adding the mdlb and not removing the exposure meter mode level is a clear engineering mistake.
• They've put the video button as a mode
  So all of a sudden, mode changes are frequent. While it was ok to stay in Av mode for ages with, e.g., a K20D, you now have to always switch between video and Av modes. If you seriously use video, you'll soon do it blindly -- and change exposure meter mode sooner or later.

So, you better regularly check your exposure meter mode, and EXIF for photos said underexposed ;)

I consider the mdlb to be a real nuisance. Pentax, please, take it back! Or make it toggle so we can leave it unlocked permanently.

And by the way, making the video button a mode is a schnapsidee (crazy idea) ...

5. Focal length info

The info screen has changed. It is more useful now. But I found no way anymore to display the focal length of a zoom lens prior to a shot.

Overall

With the exception of the mdlb, I like the new interface. And I didn't want to discuss the RAW button ;) The info button grants easy access to many quick changes. So, despite some weak points, I consider the overall button ergonomics of the K-7 even better than that of the K20D. Anyway, I really like the ergonomics of Pentax, in particular their hyperprogram and manual focus shift philosophy.

K-7 as a movie camera -- PART III: Sample video

-> Link to part I

After all the preparation, I would like to share a short and uninspired video with you. And some thoughts about post processing ...

This is a short video sequence shot at 1536 x 1024 with a Pentax K-7 during an oldtimer car meeting earlier this year near Munich.

Pentax K-7 movie in 1080p from falconeye on Vimeo.

This video is nothing special. But I will use it to illustrate

Options for post-processing

1. The form factor.

720p video is 1280x720, 1080p video is 1920x1080, both in 16:9 aspect ratio. The native video capture is in 1536x1024 or 3:2 aspect ratio.

• 1536x1024 -> 720p: magnify by 83.33%, crop 8.00% from upper and lower edge each.


• 1536x1024 -> 1080p: magnify by 125%, crop 7.81% (100px) from upper and lower edge each.

There is no benefit in recording in 720p directly, except for a better control of framing (16:9 framing on rear display) and smaller file sizes. For 720p, the camera does the same supersampling to 83% size one would do in post-processing otherwise. On the other side, keeping the 1536x1024 material provides some more options for post-processing.

1536x1024 material is not suitable for direct presentation. Here, 720p is a better option.

2. Recoding.

Most video editing software will directly open the AVI file stored on SD card. Additionally, Apple Quicktime will open the file and Quicktime Pro allows to extract individual or all frames as images. Or to recode the movie, e.g., to MP4 AVC H.264.

Photoshop CS3/CS4 can open the AVI file directly as well and you get a video layer. There, you can do many image operations like you do for still images and recode, e.g., again to MP4.

After touching up the raw material, I used Adobe Premiere Elements for more video-oriented editing. On the Apple, MacOSX has something similiar on board, called iMovie.

3. Quality.

The K-7 captures stunning video quality. But it isn't good enough to justify the extra size coming with 1080p, compared to 720p. At least not without further touching up the quality.

Below, you'll see two frame images from the short clip above. The first is as out of the camera.

The second image is post-processed using my K-7 video IQ master (a program which I developed to defeat the "768-aliasing artifact"). The K-7 video IQ master is work in progress and yet unpublished. The basic idea is to use the insight about the submatrix as described in part I and try to correct some of the weaknesses in the original algorithm as built into the camera.

You may have to click onto the images to see the original size to study the difference. According to several opinions, the filtered footage has less fringing and less jaggy lines while still gaining (or at least maintaining) on overall detail (read: without becoming softer). It was used to create the 1080p clip in the opening of the article.

I hope that a forthcoming version of the K-7 video IQ master will be good enough to render 1080p footage to the same stunning quality we now see in 720p footage. And further improve on 720p quality. Btw, I very much welcome any comments on this topic.

As a side note. It is no problem to add motion blur to frames belonging to a panning action. Or tilt to remove the skew. Just open the corresponding video sequence in CS3.

Conclusion:

This shall conclude my three part article about the video capabilities of the Pentax K-7.

The video mode in the Pentax K-7 looks like a very promising proposition. It can produce stunning video quality, esp. in 720p. However, Pentax made a number of trade-offs to keep the camera affordable as a still camera (and it is a high spec camera already w/o video). Therefore, to achieve maximum possible quality in movies, a number of tweaks applied during capture and in post-processing are of help.

Enjoy your moving images :)

Further reading:

• Ned Bunnell's blog article about using a stereo microphone
• Disussion thread about K-7 video quality
• Discussion thread about K-7 and an external microphone (dosdan's posts starting at #11)

K-7 as a movie camera -- PART II: Controlling video recording

In part I, we have discussed the technical implementation of the video feature in the Pentax K-7.

Now, let's see how to make good use of it. Again, I'm not going to repeat the specification or user's guide.

At first glance, it seems to be very straight-forward: Turn the mode dial to movie and press the shutter! Ok, done this, been there. Now, for the more serious fun: how do we control shooting parameters?

The official answer by Pentax is: you don't! There is almost no manual over shooting parameters. But as always, there are back doors :)

Things to know:

1. You can set global parameters like pixel format, quality, shake reduction in the "video" menu (press the menu button when in video mode) and is not affected by all the nice settings you may have tweaked. E.g., forget about your Auto-ISO range ;)


2. Autofocus, aperture and E/V-compensation can be set prior to a recording. All three are defunct while recording. Of course, the camera won't complain if you change aperture or focus manually (read: mechanically). The focus in video mode is contrast autofocus by default. But you can switch to phase autofocus which is faster.


3. The exposure (with all its parameters) can be locked/unlocked before and during a recording, using the AE-L button.


4. EV values in video and still image modes seem to be the same.

Controlling exposure

Knowing the video exposure response curve is key in determinig and manually controlling an exposure in a K-7 video. It isn't published by Pentax but I researched them to be this:

Assuming an aperture is preset to a given value:

Light value [EV] at f/										Shutter	Sensitivity
1.4	2.0	2.8	4.0	5.6	8.0	11	16	22	32	[s]	[ISO]
0	1	2	3	4	5	6	7	8	9	underexposure	blng red
1	2	3	4	5	6	7	8	9	10	1/30	3200
2	3	4	5	6	7	8	9	10	11	1/30	1600
3	4	5	6	7	8	9	10	11	12	1/30	800
4	5	6	7	8	9	10	11	12	13	1/30	400
5	6	7	8	9	10	11	12	13	14	1/30	200
6	7	8	9	10	11	12	13	14	15	1/30	100
7	8	9	10	11	12	13	14	15	16	1/60	100
8	9	10	11	12	13	14	15	16	17	1/125	100
9	10	11	12	13	14	15	16	17	18	1/250	100
10	11	12	13	14	15	16	17	18	19	1/500	100
11	12	13	14	15	16	17	18	19	20	1/1000	100
12	13	14	15	16	17	18	19	20	21	1/2000	100
13	14	15	16	17	18	19	20	21	22	1/4000	100
14	15	16	17	18	19	20	21	22	23	overexposure	blng red

How to read the table: Look up the aperture in the upper left (e.g., f/5.6, marked in bold), search the measured light value in the corresponding column (e.g., EV 10) and look up shutter speed and ISO in the same row (e.g., 1/30s, ISO 100).

I didn't research much how aperture would be controlled if set to AUTO. But it will choose a combination from the diagonal line of constant EV in the table above. In AUTO, the aperture is controlled live and you can hear the aperture blades moving! IMHO, this is a cool feature for a dSLR with a legacy SLR lens!

(Update) I had another look at this.

In video recording with aperture set to AUTO, I illuminated the sensor with a torch and observed the lens aperture's reaction. This is what happened:

• Without extra light: wide open (f/2.4)
• With medium extra light (torch into lens): shut down (f/5.6) (*)
• With full extra light (torch fully aligned and directly in front of lens): closed down (f/11) (*)

(*) estimated from diameter left open by aperture blades, as seen thru the front lens element.

There are really only these 3 steps. With a f/5.6 lens, this reduces to really only two steps. If it changes aperture, it does so by jumping 2EV, at least. Of course, it causes a visible jump in brightness in the video then being compensated afterwards. Therefore, manually shifting aperture creates a much smoother effect. (end of update)

Note: The above table may not be fully accurate. E.g., the sweet spot shutter speed (1/30s) may be shorter, like 1/50s, actually. Note that many videographers prefer a shutter speed of 1/30s or 1/50s (i.e., the motion blur from it) for 30fps footage to minimize a stuttering effect in panning action. Also note that all this is from my own research. Pentax doesn't disclose the information given above and the recorded video contains no useful meta information.

Now, using the response curve above, you can meter any subject, set the required aperture, use E/V-compensation to hit the required EV value, switch to movie mode (E/V compensation stays active!), lock exposure with AE-L and you successfully manually controlled your video parameters!

It is possible. But I agree that it is awkward in many if not most circumstances. If you need longer shutter times at daylight without wanting to stop down then you need to use a gray filter like you maybe would use for water still photography. Note that for video, you can stop fully down without loosing sharpness in the resulting video.

However, what I did find very easy to use is the following trick: In video mode, before starting to record, I observe the live histogram on the rear display when pointing to different subjects which will emerge during a scene. Then I point to a "typical" histogram and lock exposure. Eventually, I record using these parameters.

Controlling focus

Autofocus stays inactive during recording (it was enabled up to firmware release 0.20). However, it is so slow and badly implemented that it wouldn't be useful in actual footage anyway. If you need to refocus using the autofocus, the fastest would be to set live view autofocus to phase detect and stop a recording (press the shutter again and wait ~2s), press the AF button (~1s) and start recording again (~1s). If there is no time for a ~5s break, then you must control focus manually. Unfortunately, magnifying live view is inactive during recording as well (and the view finder stays dark, of course).

Fortunately, the quality of the rear screen with its 640x480 resolution allows an approximate focus. If the DoF effect isn't required, then shooting with fixed focus at hyperfocal distance is a viable option. The DoF calculations as obtained for an APS-C sensor do still apply. If you need to pixel peep, set circle of confusion to 0.015 mm.

If you need more control over the focus, using either a field monitor or an enlarging eye piece looking at the rear screen may be an option. The K-7 outputs live view and life audio via HDMI in 480p, 576p, 720p or 1080p. The image at the top of this article shows a K-7 hooked up to an HDMI 1.3 type C cable. However, the video data will always be 480p only, possibly enlarged to match the HDMI protocol.

K-7 videographing its own live view from falconeye on Vimeo.

The video above shows how the live feed from the K-7 behaves. In particular, you can assess the latency between reality and HDMI output. I guess it is about 1/3 s.

There are field monitors one can connect via HDMI and mount to the hot shoe or flash bar. The flash bar however, may be the better choice if a microphone is already mounted to the hot shoe ;) Because the feed is always 480p only, there would be no additional benefit in getting a 800x600 or 720p field monitor (this notice may not hold true for the Samsung GX30). A 5-6" 480p field monitor with HDMI input will do it. Avoid monitors with A/V analog input only.

Field monitors may have a headphone jack for audio playback as well.

And of course, for proper framing and smooth operation, you'll definitely want a video rig :)

Controlling shake

The K-7 features electro-mechanical sensor stabilization which proves very efficient in video capture. Note however that it is designed to work for still photography. So, it cannot compensate all the shake during a longer take. Because wide angle requires the anti shake to compensate less and is more stable in the first place, it is best to use wide angle (and tele lenses on a tripod). Or a rig again ;)

Note that a wide angle lens has a closer hyperfocal distance and produces smoother panning as well. So, I just adopted wider lenses as my standard when videographing.

Controlling audio

The built-in mono microphone is very sensitive to environmental noise like wind. So, using an external (stereo) microphone is a much better option (or use external sound recording and a take board). I tried the RØDE Stereo VideoMic connected to the flash hot shoe and it produces excellent results. The recorded quality is definitely more than sufficient for voice and sound. For music one may want to record externally.

Btw, there is no control of volume. But the recording level is relatively low and I didn't have any problems with either oversteer or noise floor.

Now, let's go out and have fun with video. I.e., it is time for part III.

-> Continue to part III

K-7 as a movie camera -- PART I: The technical foundation

One of the more exciting features of the K-7 is its ability to record video in HD.

I will not repeat the specifications here. But it has an excellent 720p@30Hz recording mode and a 1536x1024p@30Hz mode which is almost full HD and can be used to create FullHD footage. In practice, it writes MJPEG at about 50 MBit/s and therefore can often outperform video recorded in AVCHD.

Many would think that Nikon D90 was the first dSLR sporting HD video. But this is only true 50%. The earlier Pentax K20D already included the ability to record video at 1024p@21Hz, although limited to clips of 5.6s each. So, the video funtion in the K-7 is regarded as already being version 2 by many Pentaxians.

My article about video will come in three parts:

1. The technical foundation (this part)


2. Controlling video recording in practice


3. Samples videos

PART I: The technical foundation

I am the kind of person who needs to look under the hood. Driving is fun. But seeing and grabbing the engine underneath is fun too ;) Read what I've found out.

First of all, video from a dSLR isn't a trivial thing to implement. Of course, one could read out all 14.6 raw million pixels 30 times a second (or at least 24) and construct the corresponding video frames, ideally by supersampling pixels into the smaller size, and compressing to a video codec. But this implies a tremendous processing load (i.e., processing 6.53 GBit/s raw input data in real time!). Implementing it this way (and I think Canon does it this way in their 5DmkII) makes the camera significantly more expensive than would have been required by still photography alone.

Pentax choose to implement it in a way which doesn't increase cost at all. The sensor of the K20D can be read out at a rate of 750+ MBit/s, or 375+ MBit/s per each of its two channels. This is exactly enough to support 3.3 fps at full resolution. In order to support 5.2 fps with the K-7 (seemingly 6.0 fps in the Samsung GX30), Pentax/Samsung doubled the number of channels to four and the K-7 sensor can be read out at a rate of 1.5 GBit/s. As staggering as this figure may seem, it is still only ~20% of what would be required following the approach above. It would have been somewhat easier with a 6 MPixel camera though ;)

Therefore, Pentax created a special subsampling mode where only every 6th raw pixel value is read out from the sensor at 30 Hz. This special signal is always used to create live view, zoomed live view and HD video frames, for all of Pentax K20D (21 Hz), Pentax K-7, Samsung GX20 (21 Hz), Samsung GX30 and Samsung NX' electronic view finder.

Because an HD video has only 2 MPixels (or less) which is about only 1/6th of pixels of a 16:9 still image, this means that at the same ISO step, pixel noise from a video frame and from a still image will look very similiar. Of course, using only every 6th pixel looses 2.5 EV stops in low light performance compared to ambitious supersampling approach. However, even 1/6th of the surface of an APS-C sensor is still a lot larger than the combined surface in dedicated 3 chip HD camcorders. So, videos in low light will look good. It is just that they could look even better.

The subsampling matrix

How are colors reconstructed if only every 6th pixel is read? This is a problem because standard demosaicing techniques as known from a Bayer matrix don't apply. Rather, color is constructed from picking values out of the Bayer matrix with a given color filter. Below is one variant of possible locations where color values are picked from:

(note: if you click onto the image, you can find another variant which most likely would yield better results.)

As you can see, the submatrix forms a pattern repeating every 6x6 pixels. Only 6 pixel values are picked from such a 6x6 area, producing 2 RGB pixels, e.g., as outlined by the black boundaries.

To tell the truth, Pentax applies a little bit of demosaicing magic and produces 4 RGB pixels from the information of 6 raw pixels. However, it does it in a rather bad way leading to the (arti)fact that two horizontally adjacent RGB pixels have very similiar values, almost reducing the advertized resolution of 1536x1024 down to 768x1024. In part three, however, we will see that part of the information is still there. We'll call this effect the "768-aliasing artifact".

Another problem with the subsampling matrix is that colors are spatially translated, i.e., the green channel sits left of the red+blue (=purple) channels within the subsampling matrix. This leads to green fringe where contrast changes from bright to dark (left to right), and purple fringe where contrast changes from dark to bright. You can see the effect in the following image:

(400% crop of a video frame, left from the K-7, right from the K20D)

You can see the fringe effect which is exactly as wide as the 6x3 subsampling matrix (2 pixels in horizontal direction). Also, K20D and K-7 share almost the same subsampling matrix, with a bit less of the "768-aliasing artifact" in the K-7.

There is a simple experiment that demosaicing of the subsampling matrix is very rudimentary indeed: Pierce a tiny hole into a black cardboard, position the K-7 on a tripod ~10 m away and zoom into the hole image using 10x zoomed live view. With a sharp lens, at most a single raw pixel in the subsampling matrix is hit only. What you see on the rear screen, is an image of the hole which is either dim or bright, in an arbitrary color, depending on minor movements of the camera. The hole is dim if a raw pixel which doesn't take part in the subsampling matrix is hit.

So, to summarize, some fringe (false colors) and jaggy edges are artifacts resulting from the way video frames are extracted from the sensor. Note that the effects are much less visible in 720p which is supersampled from the 1526x1024 feed. The K-7's 720p video quality is stunning.

You may look at the 1536x1024 recording quality as being the "raw image equivalent" for 720p final images. Needing post-processing but retaining extra headroom.

Rolling shutter

Another common problem with video in dSLRs is the rolling shutter. Because the mechanical shutter is too slow for 30 fps, it stays open all the time and an electronic shutter is used. In the K-7, the sensor is read-out line by line in progressive order, bottom edge first, within a period of ~1/30s. Because the image is projected head down, the lines at the top of a final image have been read-out earlier than the lines further down. If the camera is panned, e.g., left to right, then vertical lines become slightly skewed, e.g., tilted anti-clockwise. Therefore, this effect is called skewing effect or jello effect as well.

The jello effect does actually bend non-horizontal straight lines if they are rotating with respect to the camera (so, avoid tilting the camera when recording). It can produce funny images with rotating structures, like propellers, actually :)

As for the K-7, this effect is very well controlled. The skew is exact and without extra jagginess. I.e., the read-out is uninterrupted. Very good! If combined with a bit of motion blur, it becomes almost invisible. I would say that the rolling shutter is almost a non-issue with the K-7. Combine this with the lack of motion-compression artifacts and a gray filter maybe, and the K-7 can produce really stunning panning action.

Video compression codec

Another strong point is the selection of high bitrate MJPEG as the compression codec. Not being an amateur's first choice, it offers greater headroom for post-processing. Individual frames are JPEG-compressed and about 200 KBytes large. The JPEG compression artifacts are visible on larger than 100% inspection but don't disturb. Also, MJPEG is an easy format for post-production. And, it doesn't cause extra burden on the in-camera processing engine.

The container is "Motion JPEG OpenDML AVI" and can be opened on all platforms, e.g., using Apple Quicktime. Of course, for long-term archival, MJPEG needs to be recoded (e.g., to MP4 AVC) to save space.

Audio

The K-7 has a built-in mono microphone capturing a clear sound in the absence of any wind.

It does have a plug for a stereo microphone too. I tried the RØDE Stereo VideoMic connected to the flash hot shoe and it produces excellent results.

It plays back audio over the built-in speaker which is really bad, though ;) For decent audio in the field, one would require an HDMI-capable field monitor with a headphone jack. Yes, I checked that audio is played back via HDMI.

The audio quality seems to be good too. The format is 1 or 2 channels of 16 Bit, 32 kHz PCM.

Temperature

After 5 - 30 minutes of continued video recording, a red thermometer pops up on the rear screen and informs about increasing temperature. I didn't have any recording stopped by temperature alert. But it may be a concern in very hot regions, full sun shine and for extended scene recording.

(Update) I've test-driven continuous video recording at 27°C ambient temperature for 40 minutes. The red thermometer appeared after about 15 minutes. But the camera didn't interrupt. The camera felt warm and burned one out of three battery marks.

A still image shot at the end was ISO 800 and 40°C. It shows a very faint vertical line in the middle, about as pronounced as the ISO800 noise and only visible against a uniform background at 100%, invisible in normal photography. Even invisible against the uniform background are additional vertical lines exactly 256 pixels apart. I don't consider this hot temperature banding to be significant considering it is from a pre-production sensor.

Other observations: right after movie recording with temperature alert, the camera refuses to enter LV but still enters movie mode and continues recording movies (or allows still images using the viewfinder). Only 20s after movie recording, it accepts LV again, incl. the red thermometer. After 2 minutes, the red thermometer has disappeared.

I think at below 30°C ambient temperature, it won't emergency-break a movie recording. (end of update)

This shall conclude Part I. Next will be a discussion about making the K-7 video feature more accessible for real projects.

-> Continue to part II