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 ;)
Very interesting read! I have a couple questions:ReplyDelete
1. Could you explain in more technical terms on the potential "loss of traction" situation you mentioned after Finding #17? I can't imagine the cause of such situation from my very limited knowledge about control theory. Thanks!
2. Do you think it is feasible to attach a test chart on the hot shoe and shoot toward a mirror, and recover the motion between the sensor and the camera body?
@Jimmy, first let me point you to two threads discussing this article right now:ReplyDelete
Ad your question 1.
The "loss of traction" means a situation where the feedback force exceeds the strength of the motor (i.e., due to limitations of the magnetic field forces) and where subsequently, the controller may try to (over)compensate for a longer period of time than necessary. As the earth's gravitation field is a constant force, it will then make a difference if the controller tries to move the sensor with or against gravitation.
Ad your question 2.
No. Because the shutter will actually not shift the body but rotate it around its center of gravity. So, the dominant rotation effect would mask any secondary relative shift effect.
Last but not least. A relative motion between body and sensor is ok. It may minimize or magnify the blur effect of the moving body. So, we would need to measure the details of that motion.
Did you test the degree to which the effect changes when shooting in portrait orientation; that is, the shutter moves left to right, instead of top to bottom as in the more commonly used landscape orientation?ReplyDelete
we did not measure this. But our 180° reversed data suggest that portrait orientation should slightly increase the effect. The increase may not be noticeable though as we talk about µm here...
Can You suggest Pentax to remove the SR system as a possible issue to blur?ReplyDelete
Or, simply do You beleive sensor SR can make things worse?
I think that way, but would like confirmation from You. Thanks
first may I ask all anonymous posters a favour? To leave a name even if it is an avatar name.
To put things straight: there may be a problem in the way how the floating sensor in the K-7 reacts to high body accelerations. But this is NOT a fundamental problem of that kind of design. A certain "soft coupling" between sensor and body is even favourable as it helps to prevent blur from sudden body accelerations. It is the particular implementation in the K-7 which could be better. I imagine it is just some nasty overlooked detail. The K20D which has almost the same SR apparatus is much better in this respect. By a factor not explained by simply assuming a more lightweight shutter.
So no, removal of the SR system isn't the fix. To fix that nasty detail is...
I am new to the DSLRs. But I am in the process to find one and the K7 is on the top of my list. I just want to get your opinion. Would you advise in the light of your recent invention still purchase the K7 or look for another cameraReplyDelete
Thanks again for the wonderful research
It's me again :)ReplyDelete
What I'm going to ask is not 100% related, but excuse me, but I think the SR mechanism only complicates the situation, already enough complicated...
Do You think, that if more robust camera (in every way) without sensor SR is made (sensor steadily fixed to the camera back), that it can have finer IQ (less blur) than the same camera with sensor shift shake reduction? Maybe this will not be possible to see with naked eye, but if using measuring instruments.....
@TJ: I can understand your hesitation. But most K-7 owners would deny the problem even exists. It really isn't obvious in practice and if you're new to dSLRs, you would have never noticed it. It's now more than a year after the K-7 came out that we could do this study. Other cameras will have different but comparable issues and 3 days ago, you wouldn't have known about this one neither ;)ReplyDelete
So yes, I do still advice the K-7 to photographers where there is match. The two weaker spots are still hispeed action (take a Canon 7D) or low light (take a K-x). The mild shutter blur is nothing to really worry about.
@Don: what can I add? Of course, SR mechanism complicates the situation. Having played with fire when we were supposed to run away (ages ago) has complicated the situation a lot!ReplyDelete
Good engineers call that a challenge. I live in the city where BMW is designed. So, don't ask me if innovation isn't possible. I would never shoot with a brick designed like a tank ...
Not to forget: free hand shake normally is the worse enemy and optical image stabilization isn't immune to shutter-induced vibrations ...
Falk, your FAQ is funny! Nice~ReplyDelete
I don't ask you any question as I have not fully digested/understood your test and report. As such, I dare not to ask you anything! ;-)
Thank You that You provided a solution for better photographs with our gear.ReplyDelete
All the best for You guys.
In your white paper conclusion you've stated:ReplyDelete
A sloppy, unstable, weak, ill behaving non-linear image sensor position controller magnifies the effect (for selected shutter speeds between about 1/180 s to 1/60 s) by another 100 %.
Is it possible for this loop behaviour to be tighened up via a firmware upgrade?
@dosdan: yes, if the control is done by the CPU as described in the patent. However, firmware symbols hint at an IC chip doing this and then it may or may not be possible without replacing this chip. Depending on the exact function of the chip. But sloppy control parameters are just ONE possibility to explain the measurements.ReplyDelete
But yes, there is a scenario where a firmware update could help. It's likelihood is another story.
Falk, The magnetic actuators that hold the sensor in position seem to be pairs of coils working in opposition to each other working on a rare earth permanent magnet. These are positioned on all 4 sides of the sensor's floating plate. The balance of current and hence flux between the opposed coils gives a null point where the sensor sits once it has leapt into position from it's resting spot at the bottom of the mirror box. These null points have been made accessible to us K7 users during live view, where we can trim the sensor position, presumably by reducing current in one coil and increasing it in the other. Perhaps if the base current through both coils could be increased somewhat (without burning out the coils or using too much battery capacity) the sensor may be held firmer in position and be able to resist the shock from the shutter. This may be only possible to implement with SR off, as it may effect the range of sensor movement with SR on. I could live with SR off at 1/80s with my 15mm lens and have the sensor held more securely!ReplyDelete
@Elmar: you make an interesting proposition. In the study's language, this would amount to increasing the maximum magnetic driving force called Fmax (below Fig.22). Not a bad idea.ReplyDelete
However, the study suggests that Fmax is sufficient. Our simulation includes the "leap into position from the resting spot at the bottom of the mirror box" as you say. This happens within a rather short period of time (18ms if I remember correctly) and our simulation delivers a minimum value for Fmax where this can succeed (driving and lock the sensor position). Using this minimum value Fmax, our simulation shows that it does also suffice to withstand the shock from the shutter-induced accelerations well enough. If the controller loop is as well behaved as the one in our simulation.
This is why we believe that it isn't a principle flaw of the SR mechanism but just an oversight for a yet to be identified detail. The fact that the K20D works well enough further supports our believe that the magnetic strength Fmax isn't simply too weak. But if, the idea may help; if feasible w/o physically changes to the camera.
Falk, Fmax would be applicable when driven hard, but how much centreing effect is there really a few uM each side of the null? Or in other terms, how broad in uM is the null point?ReplyDelete
I expect that it is not a brick wall ;)
We also have an additional bit of circuitry tacked on with the K7 over the K20 - the automatic horizon correction.
Perhaps this extra add-on has de-tuned the SR controller?
I believe that Pentax have been fiddling around with this SR controller, as I noticed that with FW17 horizons were no longer as level as I saw in the viewfinder. It's almost as if the sensor rotation factory calibration was being ignored. With FW22 it seems to be back to normal.
Just out of interest did you make any test shots with and without horizon correction? Was there any difference?
@Elmar: well, we did a full PD controller simulation taking all this into account. There is no "null point", just weaker forces for weaker deviations. Ut to Fmax for large deviations.ReplyDelete
Ignore SR, automatic horizon correction and everything. This just influences some "goal" position which is to be considered a constant during the entire shutter event. You are mixing two different time scales here.
Falk, publishing a paper which immediately includes a series of Frequently Asked Questions is illogical self referencing. By definition there can not be FAQ-s on 'day zero'. They are questions you created in your mind, not questions ASKED by the readers, and not FREQUENTLY. There were no readers yet when you released the paper (and please, do not say that many people read your paper before you published it, and you collected their questions as FAQ instead of folding the questions into the paper before the release).ReplyDelete
Do you mean something like: questions nobody asked, but some intellectually disadvantaged or not careful readers could possibly ask them, so before anyone asks, I will pretend many already did, and answer them here anticipating they will come? Hmmm...
please, refer to http://en.wikipedia.org/wiki/Humour#Humour_formula
Reading the first eight Questions again and again I fail to see anything humorous in them (the last two could be seen as mildly amusing). Possibly my engineering background blunted my sense of humor over the years :)ReplyDelete
After your response one needs to wonder: was your intention to do serious testing and write a serious paper, or is it that some sections are a humorous joke? We already know that some parts are humorous, now, which ones? It is difficult to say, unless one replicates your testing and compares the results...
Nice talking to you, Derek.
@Derek, not everybody shares the same sense of humour. To put a list of FAQ below an article when brand new is a "paradox" joke obviously, esp. if it includes questions which certainly have never been asked. Because at least one person did not get it, let me say that there are no further jokes. But believe me, the best scientific publications include the occasional joke.ReplyDelete
Thank You Falk Lumo and crew...ReplyDelete
Witty F.A.Q. anyway...ReplyDelete
Thanks for showing us the truth.
Thanks very much for your hard research, I have noticed something amiss with my K7. I previously owned a K10D which was pin sharp, and now with my K7 I have noticed blur even when shooting at 1/125 - I thought it was perhaps the fact that a higher resolution sensor needs to be held steadier and more prown to show camera shake. The dpreview forum didn't think so.....but you have answered my query, thanks.
My question, do you think if I purchase a D-BG4 grip the extra mass & stability will reduce the shutter induced blur of the K7? On the other hand, I'm afraid to invest any further in this system.
On the dpr forum, regarding my blurry images, they always told me that it is "User Error" and it is obviously camera error. Thanks
This comment has been removed by a blog administrator.ReplyDelete
GrJeck comment not passing minimum required quality for a comment.ReplyDelete
Thanks so much for this report! I had this problem with two Pentax K-x and one K-7, but not with my older K20D. Unfortunately my history with Pentax is about to end due to this blurring issue. I don't dare buying another Pentax DSLR.ReplyDelete
I have the K-x and I have noticed this in some of the pictures, always blaming the SR. Of course, it does not bother me that much, since I had the trouble maybe with around 200 images out of 25 000.ReplyDelete
Thank You very much for this research!
What the findings of this research did to me (after coming from istDS, leaving out various generations of cameras that were way better then the DS on so many levels and finally picking up that brand new K7) was constantly and panicly looking at the shutterspeed when shooting (i ususally shoot with AV mode only). It's so irrational to look out for the shutterbug in such a way but i do it because i had a very small amount of winner shots destroy by slight blurring from the old DS days. And so it's becoming a real hassle - i constantly try to avoid anything from 1/50 up to 1/125 and even then taking multiple shots because of being paranoid. Certainly i pay tribute to my controlish character but it takes the fun out of it - otherwise i would totally love this camera and don't need anything else or better for my personal photography.ReplyDelete
Thanks for your work anyway and i mean it, otherwise we would be still wondering if it's a regular user error. I for myself can't feel so 100% loyal to pentax like a couple of years ago but i guess i will stay because my investment in lenses - got myself a truly great 12-24 a couple of months ago. And i just love going fully manual K or M with my old DS.
But why o why can't Pentax come up with a primitive auto iso solution avoiding this speed region in a firmware - a maniac like me just wants a convenient workaround, i don't expect a fix or something. But i think the k7 is not in focus of Firmware Development anymore.
Hard days for true Pentaxians :(
Great study !, thank's for your big contribution to resolve problems of many Pentaxians. I finally found the reason for part of my blurred pictures.ReplyDelete
I tested the shutter blur with two Pentax k-x bodies, both are affected by the issue. The shutter blur subjectively degradates image quality a lot at 100% zoom, it is really disturbing to me. It is difficult to me to compare my results with your measurements. I think you own also k-x. I know you made no exact tests with k-x. But you surely have at least some feeling how compares k-x & k-7 regarding shutter blur. It would be appreciated to make a note about it here. Thank's
you probably use some program to calculate blur from the image. Could you advice where to find such program? Thank'sReplyDelete
@stan: About K-x, I am sorry but I didn't conduct any tests about the issue with the K-x. I may have an opinion but I hate adding noise to the internet.ReplyDelete
About blur calculation: If you follow the advice in the abstract of the paper, right before the first section "1.Motivation" starts, then you'll find all information required, incl. the software used. I could just quote the link again but using the software alone is not giving good results.