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07-21-2010, 08:36 AM - 8 Likes   #1
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LumoLabs: Shutter induced blur with the Pentax K-7 camera

LumoLabs: Shutter induced blur with the Pentax K-7 camera

Two of my friends and myself have attempted and succeeded to clarify once and forever the question if the K-7 camera can produce additional blur (compared to, e.g., the K20D). And if so, under what shooting situations. And if so, if there is a workaround. And if so, why.

We found the answers to all these questions.

Please go to
Falk Lumo blog: LumoLabs: Shutter-induced blur with an SLR camera
to learn more.
(This is no plug to create traffic towards my blog. I have no ads on my blog and don't even measure its traffic )

Some more info (skip this if you're going to read the blog article!):
--

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 Rdiger) 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:

-> LumoLabs -- Shutter-induced blur for Pentax K-7.

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 m (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 m 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 m 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.
Please, read the blog and the White paper before discussing this. Thank You.


Last edited by falconeye; 07-22-2010 at 07:09 AM.
07-21-2010, 08:43 AM   #2
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falk,

thanks for the thorough explanation.

although you stated that this study is only for the K7 and k20,
i think this may very well explain the Kx Blur phenomenon at around 1/100 and 1/125 and using wide angles.

i'll now head over to your blog to read the full study.

regards,

jordan
07-21-2010, 09:29 AM   #3
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Thanks for performing such detailed testing and presenting us with the results. Very, very interesting stuff.
07-21-2010, 10:16 AM   #4
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Falk - Thank you for taking the time to do the experiments. Since the K-7 was the first Pentax DSLR to add SR in both the horizontal and vertical directions, I feel, and your experiments confirm to me, that the issue lies therein. Simply more axes on which to counter movement. Let's hope that the next models have conquered this.

Jack

07-21-2010, 10:17 AM   #5
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seems like this is a function of the higher FPS of both cameras. k20's much less affected and i haven't notice that on my k200.

does this mean that users of the k7 (and maybe kx) are inherently "screwed" to get not-so-sharp shots between 1/60-1/125 shutters?

or is there anything that can be done in the firmware to rectify or minimize this?

bummer

Last edited by opiedog; 07-21-2010 at 10:46 AM.
07-21-2010, 12:50 PM   #6
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QuoteOriginally posted by jbinpg Quote
Falk - Thank you for taking the time to do the experiments. Since the K-7 was the first Pentax DSLR to add SR in both the horizontal and vertical directions, I feel, and your experiments confirm to me, that the issue lies therein. Simply more axes on which to counter movement. Let's hope that the next models have conquered this.

Jack
(Added bold for emphasis)

Are you sure about this? My understanding is that they all did horizontal and vertical, but the K-7 added rotational SR, which also allows the horizon correction.
07-21-2010, 12:51 PM   #7
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QuoteOriginally posted by jbinpg Quote
Since the K-7 was the first Pentax DSLR to add SR in both the horizontal and vertical directions
I think the SR was already working horizontally and vertically before the K-7 - maybe you mean rotational? Afaik that was new with the K-7
07-21-2010, 01:03 PM   #8
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QuoteOriginally posted by jbinpg Quote
Falk - Thank you for taking the time to do the experiments. Since the K-7 was the first Pentax DSLR to add SR in both the horizontal and vertical directions, I feel, and your experiments confirm to me, that the issue lies therein. Simply more axes on which to counter movement. Let's hope that the next models have conquered this.

Jack
You mean rotational, And to let you know that the K-x does not feature rotational SR. So if it is affected by the same as the K-7 then its nothing to do with the rotational SR.

07-21-2010, 01:59 PM   #9
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Wow a lot of work. Going breefly true it, reading it better in the following days. Would putting the SR to off have any influence or is that not blocking the sensor at his place?

By the way do you have the report also as a pdf-download in Deutsche Sprache?

Last edited by RonHendriks1966; 07-21-2010 at 02:30 PM.
07-21-2010, 02:37 PM   #10
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Great work Falk!
07-21-2010, 02:38 PM   #11
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2 questions about mitigating actions

Falk,
thank you and your friends for the fascinating detective work and conclusions!!!

(I've read the report, so i guess i now get to ask some dumb questions)

First of all, Wikipedia, Shutter (photography) - Wikipedia, the free encyclopedia

QuoteQuote:
Camera shake due to the impact of the larger curtains starting and stopping rapidly. Camera designers have learned to overcome the problem of mirror-slap by including a mirror-lock-up feature in some cameras. This will remove the camera-shake from the large slapping mirror inside the camera but it still does nothing to prevent camera-shake by the shutter mechanism itself. Mirror-lock-up introduces yet another problem, with the mirror locked-up out of the way you can no longer use the optical viewfinder for focussing, framing, or exposure metering. Newer DSLR cameras are now including a "live-view" where the image from the main imaging sensor is displayed directly on an LCD display, so you can still focus (manually or in newer models by contrast detection) and frame, and it prevents most camera shake from the focal-plane shutter, as instead of first curtain an electronic shutter is used
Of course, what Wikipedia doesn't address is the impact of the shutter motion on the SR behavior of Pentax like cameras. But it certainly supports Falks report conclusion that shutter motion is involved in the problem.

Questions:
1. It seems to me that Pentax's decision to go to higher shutter speeds (which of necessity would speed up the first and second curtains as a constant, i think) contributed to this problem - am i right on that?
K20D: max shutter speeed 1/4000
Kx: max shutter speed 1/6000
K7: max shutter speed 1/8000

2. If Pentax wanted to eliminate this nick blur problem, could they issue a firmware change that would reduce maximum curtain speed and of course, reduce maximum shutter speeds. (On the Kx, this would also have the likely side effect of reducing curtain noise as mentioned by Wikipedia)

3. Is it practical to use Live View as a way of eliminating first curtain vibration and mitigating this problem as suggested by the Wikipedia article. The K20 doesn't have good Live View implementation so i can't address this. I'm so used to a good optical viewfinder, not sure i would want to use Live View for framing.

Thanks again, real progress of course in understanding the problem.

Last edited by philbaum; 07-21-2010 at 02:51 PM.
07-21-2010, 02:57 PM   #12
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One "non-linear term" that might cause problem for the SR is mechanical friction in the suspension of the image sensor plate. It can very well lead to small "jumps" when the SR system wants to move the plate a very small distance.

And there are friction as we can easily test by tilting the cameras and listen to the sensor sliding around, some cameras even needs to be knocked a little before the sensor starts sliding, "Starting friction" in action.
07-21-2010, 02:59 PM   #13
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QuoteOriginally posted by philbaum Quote
2. If Pentax wanted to eliminate this nick blur problem, could they issue a firmware change that would reduce maximum curtain speed and of course, reduce maximum shutter speeds. (On the Kx, this would also have the likely side effect of reducing curtain noise as mentioned by Wikipedia)
Well, the shutter curtain is quite certainly hardware-controlled, so there is no real hope of an easy firmware fix...

What could be done by firmware would be to counter this curtain-induced sensor shake (taking into account the actual orientation of the camera), by mapping this shake with accelerometers and overlaying this to the SR loop...

As it seems to be quite repeatable and constant from one camera to the next, it could work (if the SR loop is not hardwired!), but at the expense of a huge work by Pentax...

And even then, it will greatly depend on what you have on the camera at the time, I think : additional weight (think Grip, Flash and Lens here) will surely lessen the curtain effect by decreasing the curtain/camera weight ratio...

So, as much as I'd like to see this solved on both models, I don't have much hope...
07-21-2010, 03:23 PM   #14
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QuoteOriginally posted by Gimbal Quote
One "non-linear term" that might cause problem for the SR is mechanical friction in the suspension of the image sensor plate. It can very well lead to small "jumps" when the SR system wants to move the plate a very small distance.

And there are friction as we can easily test by tilting the cameras and listen to the sensor sliding around, some cameras even needs to be knocked a little before the sensor starts sliding, "Starting friction" in action.
Really? I would be surprised if this construction is done with anything else than leaf springs (which would mean frictionless movement), but then again, I never saw a suspension system for a camera sensor. Wouldn't it just be that you don't hear it moving when it moves slowly?
07-21-2010, 03:23 PM   #15
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First and foremost: Thanks for your thorough work on this matter.

QuoteOriginally posted by Alcazar Quote
I think the SR was already working horizontally and vertically before the K-7 - maybe you mean rotational? Afaik that was new with the K-7

Actually, I do not think so. Use CTRL+F and type "rotat", at least there is claims from Pentax about 'rotational SR'. Seems to have been there from the K100D.
Pentax DSLR Shake Reduction: Image Stabilization For All Your DSLR Lenses
Pentax K100D Digital Camera - Full Review - The Imaging Resource!
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