December 1, 2010
Total blur widths as a function of shutter speed for a Pentax K-7 camera (red) and a Pentax K-5 camera (green).
Our recent study of shutter-induced blur for the Pentax K-7 SLR camera has created a lot of buzz in the Pentax community. We are now actually watching to see similiar work been done for cameras of other vendors too.
Meanwhile of course I have been more than curious to see how the Pentax K-5 camera performs in this regard. I am glad to say that the lab work is done and a report is published. You may access it here (HTML and PDF):
You may see this from the chart as depicted above as well. On average, the K-5 shutter induces a pixel blur increase by less than a pixel which should not be noticeable in day to day work. Normally, the increase is less due to other sources of softness or by not shooting at the shutter speed where the effect is largest.
Also note that every camera with a focal plane shutter (every SLR) will exhibit a certain amount of shutter blur for physical reasons.
We are not surprised to see the issue of shutter-induced blur for Pentax mitigated in the K-5. After all, the previous study for the K-7 was in reaction to numerous complaints and for the K-5, the first user feedback is very positive, incl. sharpness at the critical shutter speeds.
The conclusion cited from the white paper is this:
The shutter-induced blur in the Pentax K-5 is measurable but it should be small enough to be of no concern in day to day photography. The absolute magnitude of the effect sits halfway in between a K20D which has almost no measurable effect and a K-7 which exhibits an effect large enough to make some people notice in their work.
The matter may now have reached a satisfactory state with the K-5. But there remains work to be done for Pentax to fully understand and eliminate any unnecessary effects which compromise image sharpness.
It would be interesting to test another camera with fast shutter (like a D300s) to compare the absolute magnitude of the shutter blur effect which is never zero. Esp. at ~1/160 s. Ideally, vendors would measure it and make part of their cameras' shutter specification.
September 28, 2010
A cut model of the Pentax K-5 as shown at Photokina 2010
"The engineering masterpiece" (photo © 2010 Falk Lumo)
Pentax K-5 hands-on preview report. A short blog article turned out to be the wrong format and therefore, I posted it on my website. Here you go:
Don't miss the exciting news. Moreover, I tried to answer as many questions I received as possible in the report and to skip as much redundant info as possible too.
However, a small number of questions didn't fit the K-5 context so I am putting them here:
DA 35mm f/2.4:
The new DA 35/2.4 introduced short before Photokina is specified here:
The kit will be black only and non-black versions are said to be unavailable (at least outside Japan).
DA 18-135mm f/3.5-5.6:
This lens was announced together with the K-5 at Photokina and is specified here:
- The aperture ranges are as follows:
18-19 mm f/3.5
20-29 mm f/4.0
30-79 mm f/4.5
80-135 mm f/5.6
- DC focus motor:
"DC" stands for direct current according to one source, and for dual current according to another. That needs further clarification although my bet is with the direct current camp ;)
- Focus motor noise:
The focus motor almost certainly is not ultrasonic. I tried to listen despite the show's background noise. It sounds different than an ultrasonic motor (it's more motor-like, less twitter-like) but isn't necessarily (much) noisier.
- Focus motor speed:
The focus motor appeared to be about as fast as a DA*16-50mm ultrasonic motor. Which isn't fast exactly. The DA 18-55 WR kit lens is a lot faster!
Pentax missed a chance to deliver a higher speed focus motor (e.g. ring type) rather than a lower production cost one.
- Image quality:
I don't know. From the specs, it could be better than that of a usual floating aperture lens.
Pentax showed its 1.4x autofocus teleconverter at Photokina 2 years ago. It has disappeared. I asked what happened to it. The response I obtained was hard to accept: Pentax Germany requests production of the teleconverter but Japan denies saying that teleconverters aren't required anymore in the digital age. Pentax Germany tried to explain that a digital magnification may actually have a negative impact on image quality (as we all know) but obviously it did not help much. What shall I say...?
Post Photokina Pentax strategy (as I guess it):
Overall, Pentax is a bold performer in 2010: 645D, K-5 and K-r seem to be 3 winners. And the company itself is making profit. So what else should it take to make a Pentaxian a happy Pentaxian? Not much should one think. Esp. as an Olympus E-5 (good-bye gift?), Canon 60D (downgrade?) and Nikon D7000 (Pentax clone?) are not particularly exciting. But the grass never is green enough ;)
It's because many Pentaxians are addicted to their hobby. They want to invest further in their system and are imaginative enough to foresee "their" camera and lenses in one or two years time.
Therefore, it's a bit irritating that all signs emanating from Pentax at Photokina (foremost but not only from Pentax Germany staff) cry out: we're not going upstream with the K mount. Not now. The 645D digital medium format is as professional as one may actually desire. "Unfortunately" however, cameras in the tradition of the *istD or the higher end film bodies attracted quite a few rather professional photographers into Pentax. Over the last couple years, their fraction is shrinking though and this Photokina won't halt it. It's no fortunate coincidence either that no new Star-lenses are announced and FA Ltds. are tagged "end of life" (whatever that means...) in Germany. Personally, I also don't expect a revival of non-standard lenses (like 85/1.4, 135/1.8, 300/2.8, 600/4, maybe not even a 400/5.6), based on a statement that the rarer lenses aren't currently profitable for Pentax to make. And I hear a bold "no" to full frame. Pentax doesn't see their enthusiast user base go full frame.
To be fair, nobody knows about the future prospect of mirrored cameras. On the other hand, Pentax seems still to be in an evaluation phase with respect to mirrorless cameras and meanwhile anticipates to sell the K-r against mirrorless cameras from other vendors. This worked out fine in the past (with the K-x) but Pentax may start to realize that this won't continue for too long. So, it may be understandable that in this atmosphere of change, Pentax may not invest in the $1500+ segment in order to re-establish their share in that market segment. On the other hand, Pentax learns that what was the $1500 segment (K-7, D300) now is the $1200 segment (D7000) and soon will be in a region with aggressive price competition. As will be mirrorless cameras.
So, some kind of either upstream or innovation strategy was expected and missed from Pentax at this year's Photokina. That's the one complaint. But I hear it from many, from Pentaxians and all over the press watching the market: "where is Pentax heading?".
As a side note, there has been staff from Hoya visiting Pentax and Photokina. E.g., Ned Bunnell was spotted at Photokina and Eguchi Kazumichi (head of marketing imaging systems division) gave an interview. So, if anybody (not me) had a chance to meet with Hoya Int'l management at Photokina and exchange some ideas, please drop a comment... ;)
Summary and verdict:
- K-5: Very solid and satisfying performance. (as known so far)
- Hoya Pentax: In good shape but tight on resources. Communication skills leave some room for improvement.
I activated some ads on my blog. It enables a certain guy to pay reparations:) (just joking!)
September 20, 2010
This morning, I made a first tour across Photokina opening tomorrow in Cologne. It is already open to the press and most booths are still under construction. I'll report more about Photokina later today and this week. Nevertheless, I have a couple first impressions to share.
- Same specs as the Pentax K-7 which it succeeds, except:
- New CMOS sensor with integrated A/D converter and 16.3 MP (4928 x 3264).
- Noise reduction configurable per ISO.
- New prime II four channel processing (but I believe K-7 had prime II too).
- 7.0 fps.
- New SAFOX IX+ 11 point autofocus with higher light sensitivity (larger light intake).
"Complete rehaul and faster and more accurate".
- ISO 80 to 51.200 (standard is 100 to 12.800).
- Sensor position is adjustable in order to tune to lens (whatever this means, I will clarify).
- Full HD video with 1920x1080 25fps.
- HDR with image alignment and adjustable equalizing parameter.
- Golden rule grid and scale grid in LV.
- Electronic horizon for roll (old) and pitch (new).
- 670g body only w/o batteries.
- German list price body only €1.459,- which includes 19% VAT.
- Available in October 2010.
The press kit is full of errors and I'll have to verify a number of other details first. E.g., I doubt that the camera ships June 2009 ;) Meanwhile, I verified and it ships next month already! They fixed a number of other errors too.
The price hints at a US list price of $1,399.- which is only $100 more than the D7000 and means that both cameras will have an equal street price.
Let me repeat: The K-5 is not more expensive than the D7000!
... updated and continued from the break ...
Two points I have run a quick test for are (i.e., I may have missed a hidden custom function): no manual control in video (merde) and almost certainly no f/2.8 center AF sensor. However, the AF box on the bottom of the mirror box changed indeed. I took a photograph of it. The AF feels faster too.
"Sensor position is adjustable in order to tune to lens" and "New prime II processor" are two questions Pentax has to call back Japan to find out what this actually means ...
I'll have more about the K-5 in a separate blog article.
Pentax DA 18-135mm f/3.5-5.6 ED WR new kit lens
This lens has the additional "DC" designation. I asked. It stands for a new, faster ultrasonic focus motor. Pentax could not answer the question if this implies a ring type motor. The list price is 799.- € incl. 19% VAT.
Pentax introduces the applauded 645D medium format digital camera in the Europen market. It is the same model as in the Japanese market and will be available January 2011 for 10,000.- € incl. 19% VAT list price. Add another 1,000.- € incl. 19% VAT list price for the 55/2.8 lens.
The positioning is straightforward: best image quality for both the entire range of outdoor as well as indoor photography. What a well defined positioning ;)
I heard people say that the backlog of orders in Japan is slowly decreasing now and this is why delivery outside Japan has become possible. In other words, it was sold out which is why we haven't seen more gray imports actually.
The 645D press announcement contains no mention about a creation of a professional support network. I heard that this is under consideration though. Strange.
For anybody visiting Photokina: Pentax has one single 645D in the business area behind the public area of their booth. This is an restricted area. You normally have to be press or a business partner to enter. But insist a bit, maybe refer to my blog, and I think you will be able to do a quick test. Bring an SD card because you can put your own card if you don't ask ;)
Fuji Finepix X100
Fuji announced a camera in development, the Finepix X100. They showed a prototype and it is awesome. It is the perfect blend between a rangefinder and an EVIL camera. Well, almost because its lens isn't interchangeable.
It is an APS-C camera with a an "exceptional resolution" 23mm f/2 fixed prime lens and a very rangefinder-like viewfinder: It is both optical and electronic! You switch between optical and electronic. Additionally, the optical viewfinder may display an electronic overlay for additional information. And so, it is no surprise the camera looks like a rangefinder.
I asked why not with a lens mount? Answer: We want see how this one sells first. I asked: but you know that if this version does not sell it may be because it has no mount? Answer: no, is it so...?
Obviously, the X100 was made to compete against the Leica X1. With the optical viewfinder as its bonus. I asked if its cheaper than Leica X1. Answer was hesitant. Eventually they gave me "1000" as a ballpark number which would be about 2/3 of a Leica X1.
The Finepix X100 was one of the positive surprises of the Photokina.
Who says that Pentax is the only company with colorful bodies?
An Olympus P&S embedded into a frozen block of melting ice. Marie is behind the block though :)
Ah yes, Olympus showed the E-5 of course and a concept camera which is said to compete against the Panasonic LX5 with a high resolution Zuiko lens. They denied to answer questions about its sensor size. However to my eye, the camera looked larger than an LX5.
Lomographical bodies in various colors.
I'll update my blog with more articles about the Pentax K-5 as soon as possible. Enjoy the read for the time being.
August 6, 2010
Only a few days left until Photokina 2010
Or why at this year's 2010 Photokina exhibition, Pentax may not get
away with ignoring the full frame conumdrum.
Many users of digital SLR cameras may not be aware that the sensor in their
camera is smaller than it used to be in the era of 35mm film. Most dSLR cameras
use an APS-C sized sensor which does only have about 40% of the surface of
35mm film frame", or "full frame" in short.
Of course, this is expressed by the crop factor and users actually do know
about this. They simply ignore this. And they have reason to do so: the image
quality exceeds that of full frame film cameras and therefore, there seems
to be no reason to care about the detail of sensor size for a dSLR camera.
But as always, there is a school of photograhers who do not agree. It can
be shown that a larger sensor -- when combined with lenses which fully exploit
the design options offered by a larger sensor -- delivers superior image quality.
Independent on how great it was in the first place. After all, it's the reason
of digital medium format cameras to exist. Moreover, the lens mount of all dSLRs
with an APS-C sized sensor is desgned for full frame lenses (lenses with an
image circle large enough to use the entire surface of a full frame sized sensor).
Simply because those mounts are from the 35mm film era. And because it is the
mount (diameter and registration distance) which determines the majority of
the size of an SLR camera, a full frame dSLR wouldn't need to be (significantly)
Four-Third sized SLR cameras are an exception as their mount isn't full frame
capable. But we can savely ignore it after the impact of this sensor size on
the SLR market has almost vanished after the introduction of Micro Four-Third.
The opening photo is that of a Pentax APS-C dSLR certainly bulkier
than any full frame Pentax film SLR has ever been. Actually, the famous 30+
year old Pentax MX full frame film SLR is smaller than any digital SLR on the
market today, whatever small be its sensor. A full frame dSLR could certainly
be made more compact than the APS-C dSLR shown on the photo above.
So after dismissing the size disadvantage, we are left with only three arguments against a full frame camera which
all disappear on closer inspection, too:
Full frame sensors are too expensive and only pros
can afford it. Right?
Well, from public cost models I computed upper bounds
2010 manufacturing costs for APS-C and full frame sensrs and the difference is
less than $100 (*) ! Of course, semiconductor vendors will
add significantly different margins as long as this
market doesn't heat up.
(*) Note: for all claims made in this article, look up the "Furter reading" section at the end of the article for further reference.
The lenses are too expensive, bulky and heavy. Right?
Well, equivalent lenses (different lenses but such that
they provide equivalent image quality for different
sized sensors) have the same size and weight for APS-C
and full frame. And are a bit less expensive
for full frame! Which is easy to explain by looking
at lp/mm resolution requirements.
One needs longer, heavier, more expensive tele lenses
because one looses the crop factor. Right?
Well, about heavier and more expensive, read above. About
longer, that's actually a function of the pixel pitch,
not the crop factor. There is no reason why a full frame
camera should have a larger pixel pitch than an APS-C
camera. Currently and for cost reasons only, most actually
do indeed. Although the difference is already as small as 5µm vs. 6µm. And it is going away. Read about the cost factor above.
So, we are left with a situation where a full frame SLR
should be a no brainer. But as always, things aren't as
simple as they appear.
Because full frame cameras can deliver better image quality
they appeal to a higher segment of the market. The so-called
enthusiast and professional photographer markets. There
is a hidden consensus between Japanese camera makers (with
maybe the exception of Sony) to draw excessive margins
from the full frame market. Therefore, full frame dSLRs
are either prohibitively expensive (Nikon D3X, Canon 1DsMkIII) or crippled
in one way or another. Sometimes not voluntariliy (Sony).
And they are all bulky and ugly because they have to show
"full frame inside", right? ;)
But despite all the artificial barriers to keep the
markets segmented, more popular models like Canon 5DmkII
or Nikon D700 have risen to 2-3% unit market share each
(estimated from BCN figures). Adding the many professional models from
and Nikon and the very affordable Sony A850 we have
a unit market share of full frame dSLRs of 5-10%.
Of couse, each new model sends shock waves thru this segmentation
and an upgrade to the D700 is expected
later this year. The figures do also mean that full frame
dSLRs create a significant portion of the vendor's overall
possibly in excess of 30%.
Overall, the dSLR market is in a state like a supercooled
fluid: the right perturbation and the segmentation will
implode and all but the entry-level SLRs go full frame
(the entry-level SLRs go mirrorless Single Lens Digital
(SLD) anyway...). Think of a full frame Canon 7D or Pentax
K-7 in terms of ergonomics and speed, for well below $1800
settled street price ...
Pentax and the Full Frame mystery
Pentax is an interesting special case. They have not offered
a full frame dSLR yet and always denied any plans to launch
such a camera. On the other hand, Pentax is the brand of
the image quality perfectionist and landscape photographers.
Pentax was the first trying to launch an enthusiast full
frame dSLR, back in the year 2000 (the famous 6MP Pentax
MZ-D), three years before they launched their APS-C dSLR.
They were the first to promote weather sealed bodies and
for outdoor photography, the first to offer a 15MP APS-C
dSLR (the Pentax K20D) and they are even first to offer
an enthusiast digital medium format SLR below $10,000
(the Pentax 645D). So, they shoud be predestined to offer
outdoor 20MP+ full frame dSLR in the enthusiast market
segment (~$1200 to ~$1800). This could make
their market share explode.
But there are problems: Pentax burned fingers when trying
to be full frame in 2000. And they burned bridges when
later launching their APS-C only line of dSLRs. Because
for 5 years, they developped APS-C lenses
only and assured the market that their lens investments
are safe because full frame cameras won't come back. In
mid 2008 though, they seem to have changed direction again
and almost no new lenses have been introduced. Two new
lenses, introduced in 2008/2009, the DA*300 and the
DA*60-250/4, are patented as full
frame lenses, actually. So, who knows ... But at least
officially, Pentax is now short in full frame lenses. And
because they never left the decision between APS-C and
full frame to their customers, any announcement about a
forthcoming full frame camera could stall their on-going
business. Well, Pentax may even fear to cannibalize their
new digital medium format business which seems to have
a bright future.
Most importantly though and because they burned fingers
once, Pentax may be convinced that their share of the full
frame market is simply too small, like 5% of 5% or 0.25%
And if deciding to divert development resources, they
may as well decide to invest into the SLD market to protect
their cash cow.
So, when looking at the current or past situation, there
is no reason for Pentax to launch a full frame dSLR camera.
But what about when looking into the future?
This is were the community of Pentax photographers (called
Pentaxians by Pentax) comes into play.
Pentax and the year 2011
2011 will be the year where everybody sells SLD cameras,
Pentax included. Everybody. Pentax will have a genuine
SLD camera. As will Canon and Nikon. But where are buyers
coming from? Well, mostly upgraders from the dying point
& shoot market (in majority shifting to cell phones) and
migraters from equally priced (i.e., entry-level) dSLR.
So, to launch an SLD camera is vital to preserve a market
share in the $400 - $800 segment. Exactly where the Pentax
K-x attracted new buyers to Pentax in the year 2009. But
of this doesn't help to preserve the enthusiast (Pentaxian)
market which is the $900 - $1800 segment.
One may argue that the enthusiast market isn't that important
for Pentax from a point of view of turnover. But according
to my own analysis, K-x, K-7 and 645D each create about
the same amount of earnings. Simply because
the mass markets are always small in margin. But most importantly,
blogging Pentaxians are the most influential group: they
are opinion leaders in forums where magazine authors and
store managers draw their opinion from and recommend some
products or not. Unlike in previous years, this group
of opinion leaders has expressed very clearly that they
either expect to see Pentax going full frame in the enthusiast
segment, or leave the brand. I refer to bloggers like LanceB,
once selected "Pentaxian of the year" and who now test
drives a D700 before taking final action.
Pentax is already seeing that Pentaxians put themselves
onto "hold and wait": The sale of expensive DA lenses has
declined while the market for used (full frame) FA and
A lenses is still healthy. This isn't only due to a lack
of new DA* lenses. It is mostly due to the fact that the
enthusiast market drives lens sales and Pentaxians now
assume that the "never be full frame" claim of Pentax lost
credibility (due to technical arguments as outlined above).
After all, purchasing a lens is much more kind of an investment
than purchasing a camera which is known to deprecate
fast. The full frame conundrum is already destroying half
of Pentax' business.
So, what would be the right actions to be taken by Pentax
in 2011? Well, first, join the P&S and part of the SLR
team into a new SLD department, delivering the world's
first water-proof SLD and making sure a K-mount adapter
with AF support exists. Drop the P&S division. Pentax'
SLD mount should look as close as possible to Sony's E-mount,
35mm SLD cameras. Possibly allowing to sell Pentax lenses
for the E-mount too. I assume the E-mount uses an encrypted
communication protocol. But at least mechanically, there
should be fit.
Second though, form a team from remaining SLR and
645D developers for enthusiast cameras. Their first product
shall be a 24-35MP full frame SLR with Exmor HD
sensor in a K-5* body. Third, release new full frame lenses,
like the DFA 100/Macro relaunched earlier this year.
One may argue that this is infeasible. At least for the
second "enthusiast" team, I don't think so. Assuming Sony
would share their full frame Exmor sensor with Pentax,
all the rest is routine work for Pentax engineers. Moreover,
in June 2008, a Pentax manager said in an interview
that the development of a full frame camera is in its planning
stage. So, if that project wasn't cancelled, it should
be ready to deliver by 2011. Even taking into account that
K-7 and 645D diverted valuable resources in the first 2
years of the 3 year period. Surprisingly, the "leaked"
full frame project coincides with a rush of completed APS-C
DA lenses (2008) and a complete halt to new APS-C
only lens releases from then on and until now (with the
exception of the DA15 lens). Therefore, I guess that Pentax
would have enough full frame lenses to accompany the release
of the camera: FA31, FA43, FA77, DA*55, DA*60-250, DFA100,
DA*200, DA*300. With only 4 additional lenses (DFA28-80
kit zoom, a DFA~18 wideangle prime and two f/2.8 DFA* zooms
70-200)), the lens line up would be fairly complete. Four
lenses developped or optimized in 3 years, why not?
Pentax and Photokina 2010
Photokina 2010 starts September 21 in Cologne, Germany.
It is the event and everybody is watching
Pentax there (if at all). The internet's rumor mill has
a pretty detailed prediction about what Pentax is going
to reveal: A K-x successor (with focus indicators) and
a new camera positioned above the K-7 but not replacing
it. The first camera named K-r, the second K-5. The K-5
would be APS-C but with a 16MP Sony Exmor sensor, faster
fps (7?) and faster AF system in a body similiar or identical
to that of the K-7. At a price spot at the lower end of
the enthusiast market segment. No full frame. No SLD.
But I sincerely fear that this doesn't suffice. Starting
at Photokina 2010, Pentaxians need a roadmap to full frame
and corresponding lenses in order to be able continue to
invest into their system. They have harvested enough money
now to either invest or migrate elsewhere. And Photokina
2010 will be the checkpoint for many to take action.
To stay silent at Photokina on the full frame front may
imply suicide for Pentax in the mid term.
Pentax can wait until 2011 to say anything about SLD.
People waiting for SLD aren't the opinion leaders so letting
them wait doesn't hurt. Moreover, they can buy any other
SLD w/o leaving the Pentax system. But to disappoint the
enthusiasts now is more dangerous than it ever has been
before. Up to the point that a K-5 would only sell if the
roadmap to full frame is sufficiently clear. Which sounds
paradox but isn't. It's all about psychology and the comfort
zone for what appears to be a hobby for most Pentaxians.
Pentax has to make Pentaxians believe into the brand. That's
part of their job, actually!
So, these are my recommended action items for Pentax at
- Announce K-5
- Launch K-r
- Publish a road-map to full frame, with a first delivery
- Launch new lenses, maybe one for APS-C and two for
- Keep the SLD project a secret.
This article may be read as an open letter to Pentax.
Pentax has no plan to say anything about full frame. Which
is a severe mistake. Seize your chance. If you're too late
to the party, there may
people dancing ...
information about Pentax K-r and Pentax K-5 ff.
- Leaked information about Pentax SLD project.
frame patent of DA*300/4.
frame patent of DA*60-250/4 (note: on full
frame, the 60-250 has slight vignetting and drop of
at the tele end
which can be cured in post-processing; work tba).
leak of planned full frame project (by Shibata,
of various Pentax lenses labelled DA for full frame.
date gap for APS-C only DA lenses.
manufacturing cost of a full-frame sensor out of fab.
- About lens equivalence, article endorsed by LumoLabs.
market share by camera model in H1/2010 (no full
frame releases in this period).
- note: no information received in private communication
has been made public in this article!
You may use the commenting section to express your endorsement.
Thanks for reading!
July 21, 2010
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:
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.
- 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.
- The effect for the Pentax K-7 is larger than for the Pentax K20D. About 2 - 3x larger.
- 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.
- 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.
- 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!
- 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?
Q: Does mirror lookup work around shutter blur?
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?
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 ;)
- 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 ;)
July 14, 2010
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... ;)
June 17, 2010
We changed the layout of our site too.
LumoLabs is now at www.falklumo.com/lumolabs and hosts a repository of articles.
Therefore, we will use the blog to announce new articles or important updates to followers and interested parties. And to enable their discussion.
The actual articles are not posted as a blog article as its format was deemed unsuitable. But you'll find links to both the online article and a printable PDF version. If possible, we always recommend to download and read the PDF version. The PDF version does update more frequently too ;)
Understanding Image Sharpness
(Sample chart form the article)
Hint: The article image URLs actually open as larger images as they appear embedded in the article.
This article is a recommended read for anybody loving to dig into technology and who isn't afraid of a bit of math.
It's abstract and table of contents is:
This White Paper is one in a series of articles discussing various aspects in obtaining sharp photographs such as obtaining sharp focus, avoiding shake and motion blur, possible lens resolution etc. This paper tries to provide a common basis for a quantitative discussion of these aspects.
Table of Content
1.1. Modular Transfer Function
1.2.1. The hard pixel
1.2.2. The perfect pixel
1.2.3. The real pixel, sharp and soft
1.3. More realistic resolution measures
1.4. Combining blur
2. Sources of blur
2.1.1. Ability of deconvolution operators to reduce defocus blur
2.2. Bayer matrix and anti aliasing
2.4. Lens aberrations
2.4.1. Defocus, Spherical aberration, Coma, Astigmatism
2.5.1. Measuring shake
2.5.2. Expected shake
2.5.3. Empirical results
2.5.4. Tripod classification
2.6. Motion blur
2.8. Atmospheric perturbations
2.9. Precision and calibration
3. Practical considerations and examples
Please, proceed here:
June 10, 2010
On 2010, June 7 His Steveness has introduced the Apple iPhone 4 which features a new camera module. Two actually but I am going to talk about the main camera (on its rear side) only. And only about its still photography capabilities (it records 720p 30fps HD video as well).
It is typically hard to figure out the exact camera specification for cell phones and to compare them with a P&S camera. E.g., the HTC EVO 4G is said to have an 8 megapixel camera but little else is known. Images shot with the EVO all lack the significant EXIF information!
However, I think I managed to extract the full iPhone 4 camera specs now.
First, the pixels are 1.75 µm (according to Job's keynote) and we may assume that this is the pixel pitch. Sample images are 2592x1936 pixels (5.0 MP, 4.02:3). This makes a 4.54 x 3.39 mm^2 chip (5.67 mm diagonal) which corresponds to a 35mm film camera crop factor of 7.64.
In a P&S, this would be called a 1/3.2" sensor and P&S sensors range between 1/3.8" and 1/1.5" (crop factors between 8.6 and 3.9). So, it is an imaging sensor at the lower end of the P&S range, but it is in the P&S range and with a reasonable number of pixels and a back-illuminated sensor like the Sony Exmor R sensor. Here is a sample image from Apple's web site:
(click on the image for the sample in original size; hosted on apple.com)
Gizmodo.de has published the full EXIF data for exactly this sample photo (it is magically absent from the web sample though ...):
Color space: Uncalibrated
Date and time: 2010/05/07 15:14
Exposure: Auto exposure
Exposure time: 1/887
Flash: Flash did not fire, auto mode
Focal length: 3.85
ISO speed rating: 80
Metering mode: Spot
White balance: Auto
So, the lens is a 3.85 mm f/2.8 lens.
This corresponds to 35mm film camera equivalent lens of 29.4mm f/21.4.
I should note that even an aperture this small is not restricted by diffraction yet (limit at f/2.8 is at 54% of pixel width). This is a good value. But one really shouldn't stop down beyond f/4 and it seems the iPhone 4 has an f/3 fixed aperture lens anyway.
iPhone 4 main camera:
- 5.0 Mpixels (2592 x 1936)
- 1/3.2" back-illuminated CMOS sensor
- 4:3 aspect ratio
- 35 mm film camera crop factor: 7.64
- Low ISO 80 (or better)
- 3.85 mm lens focal length
- f/2.8 lens aperture
- Autofocus: tap to focus
Equivalent 35mm film camera and lens:
- 30 mm f/22
So, don't throw away your FourThird, APS-C or full frame SLR ;)
Besides the deep depth of field, an f/21.4 aperture at ISO 80 produces as much noise as an f/8 lens at ISO 570 (for a given shutter speed). Where 35mm film camera f/8 and APS-C f/5.6 produce the same level of noise.
The iPhone 4 seems to have a camera which should deliver an image quality on par with the lower segment of P&S cameras. It's field of view (no zoom!) corresponds to a 30mm lens (on a 35mm film camera) and it captures as much light (produces as much noise) as a system camera with a lens aperture between f/11 (FourThird) and f/22 (35mm film).
April 14, 2010
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):
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:
t0 = 0
a = 1 / (280 s)
(of course, a as above is a measurement of P. Smith's body tremor ;) )
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.
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
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.
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.
April 5, 2010
It all started when I shot the sunset panorama below from one of the locations where I work. It shows a horizontal field of view comparable to a 60mm lens on a 35mm film camera. There is nothing exceptional about this panorama except maybe that it shows -- on the righthand side -- the Zugspitze which happens to be Germany's highest mountain.
The Zugspitze is 2962 m (or 9718 Ft) high and from the point where the photo was taken, exactly 83.39 km (or 52 miles) away. So, I thought it may be a good idea to crop into that image and look what guys are doing up there on mountain top ;)
Theoretically, such a crop should be feasible because the image actually was stitched from several images taken with the FA* 300mm f/4.5 lens on the Pentax K-7 each. So, a pixel corresponds to as little as 1.4 m up there on mountain top and this excellent lens clearly outresolves the sensor.
Below is what the crop (100% pixel level and corresponding to a 2000mm lens on 35mm film) looks like:
Really looks blurry as if it were out of focus or having motion blur. Actually, razor-sharp trees in 300 m distance point out that this may play a role as well. Because 300 m isn't within hyperfocal distance for 5µm at f/8. But as we'll see, we can actually ignore this little detail.
Lesson #1: Focus on something about 1-2 miles away (i.e., which is within hyperfocal distance for a 5µm circle of confusion) because infinity may be too blurry to focus at and more nearby objects may be, well too nearby.
As it turns out, the real problem actually is that large distance objects look blurry indeed (no wonder the auto focus didn't properly lock on them).
So, I ask myself the question how sharp to expect an object at a given distance to appear?
There is scientific literature about this but I couldn't find anything accessible to photographers. So, I decided to compile a little How To guide. Starting now ...
1. Possible optical resolutions for long range tele photographs taken in the atmosphere
Of course, photos taken outside the atmosphere aren't the most important category for most people reading this ;)
Another category are photos taken from the atmosphere into outer space (astro photography) and a common figure one is finding is to expect resolutions of up to 1 arcsec resolution but no better. In nights with very low atmospheric turbulence aka as excellent "Seeing". When the stars blink less than they usually do ;) This is related to the Fried parameter r0 which is about 5cm (sea level) to 20cm (in the mountains at a very good night) large. It isn't possible to achieve better resolution than with a diffraction-limited lens with diameter r0.
A lens with diameter 300mm/8 or 38mm (<r0) isn't limited by atmospheric turbulences. However, the turbulences vary at a time scale of t0 = 0.3 r0 / v_wind and with typical values of v_wind = 2 m/s (10 m above ground) we obtain t0 ~ 1/125s.
For anything slower than t0, we effectively smear out the turbulent perturbations and decrease the resolution.
Lesson #2: Shoot at 1/125s or less, even when on a tripod :)
Of course, I wasn't aware of this and used 1/25s. But as we shall see, this isn't a big problem either. Because for excellent results, we need extremely low noise (lower than at ISO 100) and will need a long effective exposure time.
One way would be to adaptively restore turbulent distortion using a parameterized grid and stacking many restored image frames. Which is nothing but applying adaptive optics.
Another and more practically feasible way is to accept the loss in resolution due to atmospheric turbulence. But how large is it?
Well, I managed to find a formula in the scientific literature and adapt it for an optical path with constant atmospheric conditions:
for turbulent distortions along a horizontal path [%].
f: angular spatial frequency [cycles/rad].
Cn^2: (refractive-index structure coefficient),
typically between 10^-15 and 10^-13 [m^(-2/3)].
lambda: wavelength e.g. 0.55 [µm].
L: pathlength e.g. 83.39 [km].
Source: R. E. Hufnagel and N. R. Stanley, "Modulation transfer function through turbulent media", J. Opt. Soc. Am. 54, 52–61 (1964).
A public online source (i.e., free of charge) discussing this formula is I. Dror and N. S. Kopeika, "Experimental comparison of turbulence modulation transfer function and aerosol modulation transfer function through the open atmosphere", (1995).
Be f = L / (2 x) where x is the size of the smallest resolved detail. Then I derive that the limiting resolving power (i.e. where MTF drops to 5.0%) is reached where
where L1 = (1.633 * (Cn^2)^0.6 * lambda^-0.2)^(-5/8) is the distance where a 1m-sized detail can be resolved. Typical values are:
L1 = 20,000 [m^0.375] for normal turbulences,
L1 = 10,000 [m^0.375] for strong turbulences.
This formula as it stands is my own work and I hope it may be of good use for fellow photographers. The L1 values are slightly rounded (~10%) from the results using the typically rounded values for Cn^2 as given above. However, my table below uses L1 values as computed from the rounded Cn^2 values.
MTF jumps from 5% to about 40% for details twice as large (2x) and drops to below 0.01% for details half the size (x/2). So, there really isn't a reason to use more than one pixel per detail x and the maximum useful focal length can be computed from the above. The following table does so and assumes 5µm large pixels:
max. focal [mm] (5 µm pixel)
This means that you don't have to care about atmospheric turbulence if you shoot shorter than 200 m only (assuming your longest lens is 500mm).
In all other cases, turbulence may be of concern. Typically, it may not be useful to shoot more far than 1 mile away. Because you would be tempted to use your longer than 500mm which then resolves worse than 500mm.
Lesson #3: Don't shoot your 20+ Gigapixel panorama at a day with just "normal" atmospheric turbulences.
Lesson #4: Wildlife photographers wanting to resolve 1mm at a bad Seeing condition day (like in Africa) either approach to at least 100m or use 1000mm f/22 (r0!) 1/250s (t0!) which applying the Sunny 16 rule, means a tripod and ISO 400...
2. Possible optical contrast for long range tele photographs taken in the atmosphere
So far, we looked at a loss of resolution due to atmospheric turbulences. While being the strongest enemy for astro photography (besides light pollution), it isn't for long range tele photographs. While crystal-clear days exist where it is possible to view 200 km far away (on a mountain), other days clearly exist where vision is limited to a few meters only (fog).
The normal is somewhere in between where aerosol particles (due to condensed water, smoke etc.) scatter light along its path thru the atmosphere and dramatically lower the MTF with distance. The effect is much more dependent on distance than on detail size which is why we tend to not even see the object at all. Nevertheless, if we see a distant object it may be at very low contrast only. Formulas exist for MTF due to atmospheric aerosol scattering. They only mean that the useful range of tele photo lenses is limited even more.
To make things more fun, turbulence and aerosol scattering counteract each other. Dry air normally means less aerosol scattering but more turbulences too due to the heat which dried the air in the first place.
Low contrast is of double concern. Because we may wish to reconstruct missing detail which is only possible for high signal to noise ratios.
3. Improving long range tele photographs
We will apply a three step procedure to improve our tele photographs. Note that this will only be applicable for static subjects, though.
Step 1: Improve the signal to noise ratio
I took 16 images and selected the best 10 of them. Then, I used PhotoAcute to align and stack them into a "superresolved" image with a signal to noise ratio corresponding to ISO 10:
If you click onto the photo and select "Original size", you'll see that the image is twice as large. But not sharp. PhotoAcute's superresolution technique actually works for images which are sharp in the first place. Here, we only used it to boost the signal to noise ratio. Parameters used are a Nikon D40 camera with Sigma 30mm/1.4 lens, a combination I found particlarly neutral, i.e., PhotoAcute doesn't try to deconvolve for lens aberrations too much ;)
Step 2: Sharpening
The next step is a restauration of image sharpness using a deconvolution technique. FocusMagic seems to deliver best results, even in the case here where the defocus' point spread function doesn't strictly apply.
The sharpness is clearly improved. I used a blur radius of 6 pixels (after scaling the image back to 50% size). And "Forensic" regularization, made feasible by the stacking in the prior step. There is a window reflecting the sun. And because being 83 km away, it should be a perfect point. The ring artefact is a sign that a different deconvolution kernel would have yielded better results.
Step 3: Contrast enhancement
The last step is boosting the contrast within the given area. All tone values are typically within just a small range and the first step is clipping. The remaining tone mapping may be done using a gamma correction and a dose of clarity.
This resulting image may not be the most beautiful image of the top of the Zugspitze mountain. Nevertheless, from 83 km away, it not only shows a radio emmitter pole which is 4 m wide at its base and 2 m wide at its middle portion. It even shows (in the background on the right side at a 45° angle) the steel cables of the Austrian side funicular. It doesn't resolve the individual cables. But imaging them at all from more than 50 miles away is ... well, interesting ;)
Lesson #5: Burst enough images to be able to boost contrast.
Lesson #6: Stock up on a bunch of capable post-processing tools.
I hope you enjoyed the read.
- I. Dror and N. S. Kopeika, "Experimental comparison of turbulence modulation transfer function and aerosol modulation transfer function through the open atmosphere", (1995)