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-x HD video quality

The Pentax K-x is an incredible machine for an entry level SLR. Here, I will look at the quality one can expect to obtain from its 720p@24Hz HD video capture mode.

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:



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.

Watch the original at 100% size.


Zone plate test chart. 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. Watch the original at 100% size. The sampling frequency is clearly visible by the position of the four false color disks positioned at an 70% radius. The false color at and remaining artefacts beyond the sampling frequency is due to a missing video anti alias filter.

Note that the false color artifacts are significantly more intense than from the K-7, overall and esp. at 45°.

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

In comparing with the K-7, please note that the panning speed is not exactly equal. The magnitude of the rolling shutter effect is proportional to the panning speed. Also note that the K-x video has slightly more blur. Lightning condition and aperture has been the same, though.

Some testers have stated that the K-x has a stronger rolling shutter effect than other cameras. I cannot confirm this. Maybe, the opposite direction of skew disturbed the viewing habits of video testers ...


Conclusion:

The Pentax K-x delivers very good HD video quality. It has color moiré artifacts close to the Nyquist frequency which are due to subsampling without an anti alias filter. The artifacts are strong and not so nicely confined to a close neighborhood around the sampling frequency. So, in real footage one has to expect to see some textures with color moiré. This is a problem of all SLRs though, specifically those sampling at natively 720p.

The effect seems to be about that of a Pentax K-7 along horizontal and vertical lines. But the K-7 has no such artifacts along diagonal lines which may point to the fact that it uses more and differently aligned pixels in its subsampling procedure. Obviously, the K-7 scans 1024 lines (using 864 in 16:9 format) whereas the K-x only scans 720 lines.

Interestingly and unlike the K-7, the K-x has no magenta/green fringing and no double seam effect along near vertical lines. The subjective effective resolution in 16:9 is about 1350x717 or 1.0 MPixel. This is an excellent value not achieved by consumer HD camcorders. It even surpasses the 1280 limit ;) The K-7 has slightly less resolution in the horizontal direction but much better resolution along diagonal lines.


Further reading:
- Lumolabs: Pentax K-7 HD video quality
- 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.

Lumolabs: Welcome and testing methodology

Welcome to Lumolabs.

I never was really satisfied with the big lab tests (dxomark.com and dpreview.com) and really was disappointed by print magazine and other ezine tests. So, I decided to set up my own testing laboratory, based on experience as a photographer, insight as a physicist, software writing and image processing skills and patience.

For the time being, I am not challenging the big labs and confine my ambitions to the following areas:

1. RAW file image quality, in particular noise, dynamic range and resolution.
2. Video frame image quality, in particular resolution and artifacts.
3. Pentax.

I am not looking into color accuracy (not even chrominance noise...) or the camera's JPG engine. Or autofocus, ergonomy, build quality, etc. I may do lens tests and other vendors later as well.


Let me provide you with a short summary of my testing methodology:


RAW file treatment

I use the RAW file from the camera (DNG preferred) and use Adobe Lightroom 2.4 (LR) to do the demosaicing. The standard settings must be altered in the follwing ways to get a neutral demosaiced 16Bit sRGB TIFF file:

- Gray card calibrated white balance (I use 2900°K halogen tungsten light).
- Blacks 0 (changed from 5)
- Brightness 0 (changed from 50)
- Contrast 0 (changed from 25)
- Sharpening 0 (changed from 25)
- Noise reduction Color 0 (changed from 25)

I then read the resulting 16Bit sRGB TIFF file into my own lumolabs software. I properly convert the sRGB response curve (which isn't exactly a gamma curve) into linear colors and 18% gray is RGB 117.6/255.


Taking test chart photos

I then take test chart photos with controlled manual settings, including manual focus. About the slowest shutter speed is 1/15s.

Some think that noise test shots must be taken in low tungsten light. I agree for the tungsten part and I use it. I disagree for the low light part, though. A short exposure is a perfect approximation to low light because the read-out time of the sensor is longer than 0.1 seconds anyway whatever fast be the shutter. So, only if I wanted to study exposure time longer than, say 1/5s would low light be able to have an influence. In consequence, I don't publish exposure times shorter than 1/10s. Additionally, except for initial focus, I avoid using live view in noise tests to not overheat the sensor.

One of my test charts is the zone plate chart useful to evaluate anti alias filter and Bayer mosaic filter artifacts. It is shown in the beginning of the article. It is allowed to download it for own testing purposes, providing proper credit is given.

Other testing charts are an ISO-12233 resolution chart which I modified to include inner parts of 4x the resolution. Another chart is the noise testing chart with defined gray and color patches. All charts are printed on A2 paper using a high end photo printer. There are minimal printing artifacts visible at and beyond 4000 LW/PH (line widths per picture height). They do not disturb the testing results though.

The density range of the printed noise test chart is about 7 EV. So, I take a second shot at -5EV and obtain a combined test chart of about 12 EV dynamic range (luminance range 0.02% to 100%). Which immediateley turned out to be outperformed by the recent Pentax K-x ;)

My software will search for patches of minimal variance to exclude the effect of scratches and the like which may be present on the printed test chart. I am not sure though that there aren't some invisible defects below the -40dB range. They do not disturb the testing results though, except maybe for very bright patches.


SNR, noise and dynamic range

I am unrelated to DxO Labs, Boulogne, France. However, whereever I found their methodology to be appropriate (cf. dxomark.com), I adopted it. I disagree on some of their methodologies but some are best practise to be followed. My biggest concern with DxO is their lack of examination of influence of spatial frequency dependencies. They only "correct" their results for resolution effects as to be expected from pure photon shot noise. IMHO, this isn't good enough. I am currently working at a new testing methodology to overcome this. For the time being though, I am happy to adopt DxO's pixel-level measure methodologies.

It is therefore important to understand the SNR [dB] figure. It is explained here: SNR.

Another important detail is the print normalization: Noise, dynamic range etc. are all improved by downsizing an image. Still, measures take place at the varying resolution of the sensor. Like DxO, I use a "print-normalization" calibrated to a 8 MPixel resolution. The normalization formulae are here: print-normalization.

SNR [dB] can be easily understood by using the follwing hints:
- A patch of basically random pixels has an SNR of 0 dB.
- A decrease of light by 1 EV, doubling the ISO, going to a 50% darger region in an image or doubling the number of pixels all amount to a change of SNR by -3dB.
- Going from 1.53 crop APS-C to full frame amounts to a change of SNR by +3.7dB. So, 3dB is a lot ...

My full SNR curves can be directly compared with the DxO mark tab "Full SNR". Note however that DxO uses noise patches made from glass, has a different light source most importantly at a different color temperature and uses a different raw converter. Also, in derived measures, I don't include ISO sensitivity variance and possible raw data noise reduction (smoothing).

The dynamic range is the luminance level where SNR reaches 0dB, converted to f-stops and normalized for resolution. Note that the 0dB level is arbitrarily choosen by DxO and is resolution dependent. They formula-correct for resolution dependency but fail to understand that the full dynamic range must be measured at a much lower resolution. I plan to address this issue.


Video frame tests

The video frames studied in the video section are extracted using the Quicktime Pro "export as an image" feature. Because the video uses the camera's JPG engine, here are the parameters:

- Pentax: WB manual (2900°K), "Natural color profile preset", all other parameters at default values (like noise reduction and sharpness -1).


Enjoy the tests :)

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.