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Author Topic: IQ reduction with higher shutter speeds?  (Read 6782 times)

mreco99

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IQ reduction with higher shutter speeds?
« on: November 14, 2011, 02:59:14 PM »
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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IQ reduction with higher shutter speeds?
« on: November 14, 2011, 02:59:14 PM »

neuroanatomist

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Re: IQ reduction with higher shutter speeds?
« Reply #1 on: November 14, 2011, 03:15:45 PM »
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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Re: IQ reduction with higher shutter speeds?
« Reply #2 on: November 14, 2011, 03:21:55 PM »
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.

mreco99

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Re: IQ reduction with higher shutter speeds?
« Reply #3 on: November 14, 2011, 04:17:22 PM »
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.


thepancakeman

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Re: IQ reduction with higher shutter speeds?
« Reply #4 on: November 14, 2011, 04:47:01 PM »
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)?
« Last Edit: November 14, 2011, 05:27:04 PM by thepancakeman »

dr croubie

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Re: IQ reduction with higher shutter speeds?
« Reply #5 on: November 14, 2011, 05:25:04 PM »
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

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Re: IQ reduction with higher shutter speeds?
« Reply #6 on: November 14, 2011, 05:54:00 PM »
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?
« Last Edit: November 14, 2011, 08:32:44 PM by mreco99 »

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Re: IQ reduction with higher shutter speeds?
« Reply #6 on: November 14, 2011, 05:54:00 PM »

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Re: IQ reduction with higher shutter speeds?
« Reply #7 on: November 14, 2011, 06:32:41 PM »
that was an extremely interesting article thanks neuro!
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smeggy

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Re: IQ reduction with higher shutter speeds?
« Reply #8 on: November 14, 2011, 06:51:14 PM »
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?
« Last Edit: November 14, 2011, 07:11:39 PM by smeggy »

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Re: IQ reduction with higher shutter speeds?
« Reply #9 on: November 14, 2011, 07:21:18 PM »
Great article.  Shows how technology is eroding fundamentals.
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Re: IQ reduction with higher shutter speeds?
« Reply #10 on: November 14, 2011, 08:49:47 PM »
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

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Re: IQ reduction with higher shutter speeds?
« Reply #11 on: November 14, 2011, 10:50:56 PM »
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

Edwin Herdman

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Re: IQ reduction with higher shutter speeds?
« Reply #12 on: November 15, 2011, 01:36:01 AM »
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?

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.

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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.
« Last Edit: November 15, 2011, 01:47:36 AM by Edwin Herdman »

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Re: IQ reduction with higher shutter speeds?
« Reply #12 on: November 15, 2011, 01:36:01 AM »

smeggy

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Re: IQ reduction with higher shutter speeds?
« Reply #13 on: November 15, 2011, 06:28:04 PM »
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.

mreco99

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Re: IQ reduction with higher shutter speeds?
« Reply #14 on: November 16, 2011, 03:35:13 AM »
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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Re: IQ reduction with higher shutter speeds?
« Reply #14 on: November 16, 2011, 03:35:13 AM »