# IQ reduction with higher shutter speeds?



## mreco99 (Nov 14, 2011)

Hi

Is it possible to have any loss in image quality soley by using the highest shutter speed (ie 1:6400) assuming ISO and everything else was equal?

I think the answer is no, but a couple of my pictures give me reason to think otherwise.

thanks


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## neuroanatomist (Nov 14, 2011)

Maybe. It depends on the lens - if you are using a lens with IS, and the IS is on, there may be a slight negative impact on IQ with fast shutter speeds. Thom Hogan has a write-up on it, as it pertains to the Nikon VR system, but the same principles apply.


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## Mt Spokane Photography (Nov 14, 2011)

Why not post them along with all the info about how they were taken. 

There may be some things that can affect IQ such as having IS turned on as Nuero mentioned.

A failing shutter is another one of the possible issues, it usually starts to impact images at high shutter speeds.


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## mreco99 (Nov 14, 2011)

thanks guys,
I havent got any concrete evidence to show. But it was with IS on my 70-200 mk2 (at 200) and the 5dmk2, only a week old so i hope its not failing shutter 
Its probably me, but i wondered.


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## thepancakeman (Nov 14, 2011)

neuroanatomist said:


> Maybe. It depends on the lens - if you are using a lens with IS, and the IS is on, there may be a slight negative impact on IQ with fast shutter speeds. Thom Hogan has a write-up on it, as it pertains to the Nikon VR system, but the same principles apply.



Read the article--very useful, thanks! 

One thing that still didn't seem clear to me: if you are shooting from a stable surface (like the ground) at a high shutter speed (let's say 1/1000+), but are panning with a moving subject (30 mph cyclist) should you or should you not use IS, presumably in Mode 2 (panning)?


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## dr croubie (Nov 14, 2011)

mreco99 said:


> Hi
> 
> Is it possible to have any loss in image quality soley by using the highest shutter speed (ie 1:6400) assuming ISO and everything else was equal?



Unfortunately, there's not really any way to do a real scientific comparison side-by-side. Chaning from a higher to lower shutter, using the same ISO and same Aperture (which both influence IQ), you can only really add an ND-filter (which can influence the IQ), or have a really well-controlled studio-lighting setup that you can turn up/down exactly to match the change in shutter speed (but then you need a flash synchronisation of 1/6400s, not even leaf shutters do that).

The IS is a known-issue, however slight, and that could be the cause of your problem (if you're at 1/6400s, then you definitely only need the IS for easier framing, if not, turn it off).
Or do you know what your shutter count is? (only ways to check are just to count how many times your IMG_x has gone clicked around to 0 again, or find a friend with Linux and Gphoto)


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## mreco99 (Nov 14, 2011)

yes neuro , very good article, with much enlightenment.
Out of interest, what shutter speed do you think you can get away with (with no IS) to properly freeze a average flying bird?


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## wickidwombat (Nov 14, 2011)

that was an extremely interesting article thanks neuro!


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## smeggy (Nov 14, 2011)

This is a sincere question ...

Could the problem be diffraction?

Here's what I'm thinking.
With slower shutter speeds, most of the light (timewise) that falls on each of the pixels won't have been near the shutter blades, hence the issue doesn't dominate.
At higher shutter speeds, the front and rear curtains of the shutter will fall close to each other, with a spacing of a few mm, or even less than 1mm by my reckoning. Thus the light that falls on the pixels would always have been near a blade. From what I remember, this is getting close to the danger zone.
There will be a point where diffraction must dominate, resulting with vertical smearing. I don't know where that level is, but given the effect of diaphragms in lenses, I suspect the level cannot be that far away.

Does that make any sense, or am I barking up the wrong tree?


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## DanoPhoto (Nov 15, 2011)

Great article. Shows how technology is eroding fundamentals.


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## Halfrack (Nov 15, 2011)

thepancakeman said:


> One thing that still didn't seem clear to me: if you are shooting from a stable surface (like the ground) at a high shutter speed (let's say 1/1000+), but are panning with a moving subject (30 mph cyclist) should you or should you not use IS, presumably in Mode 2 (panning)?



IS is pretty much for 1/250 and slower shutter speeds, and is more about preventing shake/slap from impacting the image. If you're shooting from a tripod, turn it off. If you're following a subject - you'll want IS off and the higher shutter speed / ISO to freeze the action.


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## mreco99 (Nov 15, 2011)

mreco99 said:


> yes neuro , very good article, with much enlightenment.
> Out of interest, what shutter speed do you think you can get away with (with no IS) to properly freeze a average flying bird?



self answered
http://www.shutterfreaks.com/Tips/FlyingBirds.html


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## Edwin Herdman (Nov 15, 2011)

smeggy said:


> This is a sincere question ...
> 
> Could the problem be diffraction?
> 
> ...


You are onto something, and no, you are not barking up the wrong tree (although in my case, these hibernating winter trees aren't so quick to absorb the facts)! Look here to Joseph P. Wisniewski's post:

http://forums.dpreview.com/forums/readflat.asp?forum=1018&message=34340728&changemode=1

You'll note that, for the figures given there, the shutter blades are actually closer together than you had predicted, but no matter. What's more troublesome is that these shutter speeds for an idealized camera, and the more practical notation of what distances are equivalent to what apertures (i.e. f/5 and f/12) are not true for any camera that is not exactly like these. You'd have to figure out, in other words, how far apart the aperture blades of a given camera actually get.

_____________________

And of course I wasted a huge amount of time thinking about a response to a question that wasn't asked. In case anybody was interested for my starting point (by no means definitive) about how IS and diffraction may be related (ignoring the interesting possible complication of microlenses, which smeggy's post mentions in passing):

Just to briefly recap what IS is: IS takes a lens or group of optical lenses in the barrel of the lens and moves it in response to gyroscopic readings in order to slightly shift the image back and forth, with the goal of moving a bundle of rays back to the center (even though the part of the scene those rays originate from is now off-center from the front of the lens).

And diffraction is the result of light encountering obstacles. In the above scenario, a moving IS element would seem, in a simple thought experiment, to bend the rays to a point where they encounter an obstacle - the shutter blades, in the case you mention. However, this would depend on where in relationship to the shutter blades the IS group is.

First, however, forget the shutter blades and consider that if the IS group shifts, the edge of the IS lens group that previously was able to collect light from the image could theoretically be blocked by the physical edge of the lens groups (possibly "optical vignetting") or the barrel in front of the IS group ("mechanical vignetting"), because the IS group is trying to "catch up" with the image that has moved relative to the front of the lens (camera moves = lens moves = image moves), and in our theoretical system it may not stop trying to compensate even when the image has moved outside its range. In reality, IS systems do not try to pursue an image that has vanished from the lens' sight. In both cases, the result would simply be vignetting, which is already a problem in many prime lenses, let alone complex zoom lenses with IS. This poses no problem through the rest of the image.

Assuming that IS optical lens groups would logically come before the shutter blades to prevent shutter blades from adding yet more interference woes to the optical and mechanical vignetting, I believe that the only part of the rays from outside that would encounter an obstacle will do so before (or possibly when) they reach the IS group - so the IS group may be slightly larger than needed in order to reduce this obstacle, and the same may be true of elements in front of it.

After the IS group is aligned, the rays will now continue straight down the lens where you can assume normal diffraction effects will set in (i.e., will be determined by highly restricted aptertures).

Diffraction effects can be controlled by the aperture, and in our equation for exposure (Exposure = Light (assuming constant radiation) * Time) we see that the directly related variable, light, is separate from time altogether. The variable most obviously related to IS or VR or whatever, Time, does allow you to vary the light, however.

As a result, if you are shooting beyond the point where diffraction starts to set in for your camera and lens combination, you may find that IS or VR can be utilized to lengthen the exposure. The result is a shot that's more free of shake and appears to have the same brightness, at the cost of more time taken - and in this case, preserves a small aperture.

It may well be the case that VR, when combined with a camera's tendency to overexpose some scenes to meet its internal "this scene is 18% gray like most others" assumption, and with Auto ISO, is allowing some photographers to negligently shoot at apertures that are diffraction limited without their understanding what they are doing. Alternatively, they could be using shutter speeds that are much too long for the scene brightness. I can't imagine at the moment that any cameras will take a bright scene and automatically throttle it down to f/11 or whatever is obviously diffraction limited, though it seems possible - that would have to be a really bright scene! Most peoples' memorable experiences with their camera's programming obsession with scene brightness will result from the opposite situation - when the scene is too dark and the camera ramps up ISO and opens everything else up as far as possible.


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## smeggy (Nov 15, 2011)

Edwin Herdman said:


> You'd have to figure out, in other words, how far apart the aperture blades of a given camera actually get.


I just went one better: I measured them 

An hour of high speed flash photography and hundreds of shutter activations later:
(using the size of imager [mirror up] as reference)

1000D at 1/4000 = 1.5mm

5D at 1/8000 = 1.7mm

Clearly the "dwell time" mentioned in the article you linked Edwin has a considerable impact on the operation. Some of my "Sync Timing" tweaking with the PocketWizards are testament to this.

Given the distance from the shutters to the imager (5mm might be right) and the resulting proportional increases in focal ratio (to ~F/4) you would think theses wider apertures would poop all over my diffraction theory.
However, we should consider that this is an iris (slit) in terms of time, not distance (the latter being for a conventional lens aperture).
During these short exposures, each pixel will experience nothing at all, then strong diffraction, then weaker diffraction, then *one instant of F/4*, then weak diffraction, then strong diffraction, then nothing.

So the net effect will be diffraction considerably worse than F/4. Again, I don't know what the overall level of diffraction will be, but I suspect the diffraction could be getting close to having a significant effect.

I will try some more tests when I can borrow my friend's 7D this weekend; 1/8000 and high pixel density should be a good combination.


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## mreco99 (Nov 16, 2011)

As much as i love all the techno chat, do you think you could also end the long technical bits with simpletons summarys, for us thickos  thanks


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## smeggy (Nov 19, 2011)

Initial results with a 7D (and 100mm F2.8 IS *L* Macro), taking photos of fairly distant grass, LiveView focus (then auto focus completely disabled between photo sets) â€“ and *IS ALWAYS OFF*:

1/1000, ISO 500, F3.2 = comparatively sharp.
1/8000, ISO 4000, F3.2 = considerably blurry.

For that result, I thought the ISO setting could have had a significant impact (the noise taking the limited compression bandwidth away from the detail of the image), so for a next test I compensated the exposure by instead changing the aperture:

1/1000, ISO 4000, F10 (I didnâ€™t want to go too far) = comparatively sharp.
1/8000, ISO 4000, F3.2 = fairly blurred.

Image noise obviously isnâ€™t the issue.
If there was nothing to the theory of â€˜slit diffractionâ€™ then the F10 image should have been slightly worse. So it seems there is merit to the original poster's suspicions.
I will try some proper diffraction testing later on.


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## epsiloneri (Nov 19, 2011)

smeggy said:


> During these short exposures, each pixel will experience nothing at all, then strong diffraction, then weaker diffraction, then *one instant of F/4*, then weak diffraction, then strong diffraction, then nothing.



Don't forget that the shutter is close to the image plane and not in the parallel beam - it makes a big difference, because you're only looking at a fraction of the image at a time. An aperture of 1.5mm would give a diffraction at 550nm wavelength of ~0.038 degrees. At 5mm distance from the image plane (let's say), that corresponds to 0.67 micrometer, 10 times smaller than a typical sensor pixel and thus completely negligible.

What you say about "weak-strong-weak" diffraction is the reason large apertures show less diffraction in the first place. It doesn't really matter that you have a temporally changing diffraction pattern. Technically, you can compute the (1D) diffraction pattern from the slit and convolve the "true" image with the resulting transmission function to get the expected detected image.



smeggy said:


> 1/1000, ISO 4000, F10 (I didnâ€™t want to go too far) = comparatively sharp.
> 1/8000, ISO 4000, F3.2 = fairly blurred.



It's no good changing F-number, because lens properties like sharpness typically change with it. It is also not good using high iso, because of resolution becoming limited by detector noise.

Instead, I suggest you do the following test: Photograph a test chart (or whatever) in bright light and short exposure (say, f5.6 and t = 1/8000 s and appropriate iso). Wait until it gets dark (or make it dark), then shoot the same chart with flash and synch t = 1/125, f5.6, same iso (and E-TTL II). This will ensure that the shutter blades do not interfere at all.

Or, simpler: use ND filters. Even if they did interfere with the sharpness, it would go in the wrong direction (making the slower exposures more blurry, because they have ND filters). Since they are mounted in the parallel beam, they really do not change sharpness. The only potential problem is flares.


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## epsiloneri (Nov 19, 2011)

neuroanatomist said:


> Maybe. It depends on the lens - if you are using a lens with IS, and the IS is on, there may be a slight negative impact on IQ with fast shutter speeds. Thom Hogan has a write-up on it, as it pertains to the Nikon VR system, but the same principles apply.



It was a long text, but the description of _why_ IS would impact IQ at high shutter speeds was not intelligible. So the VR sampling is 1000 Hz, meaning VR cannot resolve faster shutter speeds than 1/500s. The only thing that tells me is that VR doesn't really help IQ at those shutter speeds, I don't understand how it can make the IQ _worse_. What is the relevance of the image only being exposed one part at a time, and why does it matter that VR sampling and shutter action are correlated, if that's indeed the case?

Without a credible explanation, I don't find anecdotal evidence very convincing. Controlled experiments (IS on/off ta high shutter speeds) should be trivial to make, and that there is no reference to such makes me very suspicious indeed.


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## mreco99 (Nov 19, 2011)

thanks for all the efforts gone into this post, i am still reading with interest...


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## epsiloneri (Nov 19, 2011)

dilbert said:


> Does that help?



Ok, the IS produces image motion during 1/2000th second. But then why doesn't it during 1/500th of a second? And why isn't this trivial to test on an IS lens?


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## smeggy (Nov 20, 2011)

I have done basic diffraction testing. A modified white LED (lens portion removed), with a thin black cover with a 0.6mm hole drilled into it, 4m away from the 7D with a 17-40mm L lens on it, helps ensure the imager sees a point source of light so making any effect of diffraction obvious. I varied the current though the LED to compensate the change of exposure time. Obviously, the ISO and aperture remained unchanged.

Results: nada!

I had to greatly overexpose the image (very high ISO) in order to see the effect of diffraction; only then could I see the tell-tale, sideways-Saturn-looking vertical spikes around the point (a few pixels) of light. So the diffraction effect is present, but it is certainly isnâ€™t dominant. So I think my theory is â€œBustedâ€.
So why my previous second result? I think I owe folks an apology. I suspect my second test was subject to a slight amount of focus error (even with LiveView, go figure!), hence it affected my interpretation of the result. As most folks here should know, stopping down can sharpen the out-of-focus portions of an image <kicks self>
It has been a really crap day here in London (really thick fog all day), so I couldnâ€™t confirm that result (or do any other IQ testing).

I suspect the IS doesnâ€™t make any significant difference if the exposure time is so short. I canâ€™t see how the IS mechanics could accelerate within such a short amount of time; this is regardless of sample rate, but if the sample rate is really is up to 1000 Hz, then the exposure time is much shorter than the sample interval. So I reckon if IS was the problem, any negative contribution should be proportionally worse with longer exposures as the processing still wouldn't be updated in that time.

mreco99,
How did you take images with different exposures, yet keeping all else unchanged? Were the exposures compensated in other ways, or were the resulting images of varying brightness? Due to the non-linearity of the photon to bitcount conversion (the â€˜gammaâ€™ if you wish), it is not impossible that the contrast of the images was different; perhaps this could be interpreted as a loss of IQ/sharpness.


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## Edwin Herdman (Nov 21, 2011)

smeggy said:


> Edwin Herdman said:
> 
> 
> > You'd have to figure out, in other words, how far apart the aperture blades of a given camera actually get.
> ...


Nice!

The differences in spacing between the 1000D and the 5D are interesting - and in the direction I'd have expected. Although I'd like to know what the 1/4000 sec distance is for the 5D (3.4mm? Am I discounting geometry here?)


smeggy said:


> I suspect the IS doesnâ€™t make any significant difference if the exposure time is so short. I canâ€™t see how the IS mechanics could accelerate within such a short amount of time; this is regardless of sample rate, but if the sample rate is really is up to 1000 Hz, then the exposure time is much shorter than the sample interval. So I reckon if IS was the problem, any negative contribution should be proportionally worse with longer exposures as the processing still wouldn't be updated in that time.


Agreed. I'm having trouble trying to visualize a situation where the varied frequencies of the shutter and the IS update would destructively interfere with each other. We also know that the longer the shutter speed gets, the better the effect of IS versus an unstabilized image (for handheld, non-tripod use, of course).


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## epsiloneri (Nov 22, 2011)

mreco99 said:


> Is it possible to have any loss in image quality soley by using the highest shutter speed (ie 1:6400) assuming ISO and everything else was equal?



"Impossible" is a strong word, but I tested exposures 1/2000 & 1/8000 seconds with EOS 7D and EF 70-200/2.8 IS II @ 200/2.8 and EF 400/2.8L IS @ 400/2.8, both handheld, shooting a test chart at 3 meter and 6 meter distance, and noticed *no difference* in IQ (as expected really). The exposure level was made equal by moving the studio light (distance difference a factor of 2 gives illumination difference of 4).

I also tested the claim that IS could deteriorate IQ at high shutter speeds by switching the IS on/off, and noticed *no difference* in IQ under these conditions. I made sure there was no AF error variability by reacquiring AF multiple times in each setting.

I would like to try with a smaller aperture as well, but as even my bright studio lights are not sufficient for these high shutter speeds, I will have to wait 5 months for some decent daylight to test it.


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## smeggy (Nov 22, 2011)

Edwin Herdman said:


> The differences in spacing between the 1000D and the 5D are interesting - and in the direction I'd have expected. Although I'd like to know what the 1/4000 sec distance is for the 5D (3.4mm? Am I discounting geometry here?)


What you say makes sense. Given that speed of the shutter blades never change for a given part of the imager (not affected by exposure time), the gap between the blades should be in proportion to the exposure time (when both are simultaneously in motion).

However, I realised another important factor: the acceleration of the shutter blades. Being as the shutter is a simple spring-loaded mechanism, I strongly suspect the blades donâ€™t immediately get to speed and then hold that speed; the speed of the blades should constantly increase as they fall. So at the start of the exposure (the gap at the top of the imager) I think the blades should a closer together than at the end (gap at the bottom of the imager). Again I donâ€™t have a quantitive feel, but I reckon this factor wonâ€™t be of any significance. I will try to measure this later.


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## mreco99 (Nov 24, 2011)

So to sum up (so far) we havent actually got any hard evidance conclusions?


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## epsiloneri (Nov 25, 2011)

dilbert said:


> When the shutter speed of the camera is (say) 1/100 of a second when IS is active, the lens can have moved the image back and forward from a single point 10 times during the exposure and it will appear solid. On the other hand, if the shutter is set to 1/2000 of a second, you'll capture the lens moving the image between two points.



Thanks for the explanation, Dilbert... but it still doesn't make sense to me. Let's say that for a slow exposure IS moves between positions A and B sufficiently many times that, effectively, the image is superposition of IS at A and B. On the other hand, with a very fast exposure, you may catch the IS while moving from A to B. This is what you are saying, right? But then why would the second case be _less_ sharp than the first case? Why is moving between A and B less sharp than showing A and B simultaneously? 



dilbert said:


> To think of it in a different way... our eyes are able to respond at around 10 to 12hz. If you look at a CRT that has a frequency of 50 or 60hz, you see a solid image. If you take a photo of that CRT using a camera set to (say) 1/100 of a second, you get a less than solid picture on the CRT.



The reason for this is that a CRT draws only part of the screen at the same time, while other parts of the screen quickly fade to black until updated in the next cycle. The equivalent would be to shoot a scene that is alternating to black at high frequency. Then yes, it could make sense to average over a couple of cycles, instead of catching it while in the black phase. But that is not what happens with IS. 

You should instead compare to an LCD screen, which shows the same picture until updated (also at 60 Hz). If you expose for short enough time you may catch the LCD during an update and get part of the screen showing a frame and parts of it showing the next frame. With a longer exposure you may get both or more frames. There is no big difference for the IQ in this case.



dilbert said:


> > And why isn't this trivial to test on an IS lens?
> 
> 
> Because it's not trivial to come up with a lighting source that illuminates your subject such that you can shoot it at 1/2000 or 1/1000 at with the camera/lens set for good IQ (ISO 100, f/8, etc.)



I understand that, because it requires bright conditions. But if it's hard to come up with even a test case where IS is a problem for short exposures, then it probably isn't a problem. And if you, ever, get sufficient light that you can use ISO 100 with f/8 and a very short exposure, then it should be trivial to switch between IS on and off, with everything else being the same, to confirm the potential IQ degradation introduced by IS.


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