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Author Topic: Strange bokeh (out-of-fucus) with EF 600 f/4L IS  (Read 51303 times)
Mikael
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« Reply #40 on: May 20, 2008, 04:02:36 AM »
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Obviously, they did not understand the question at all. They have no idea what you're talking about. They think you're talking about shake-induced blur and whether the stabilizer itself can create (rather than defeat) blur. They also refer to over-corrected spherical aberrations which is a different kettle of fish entirely.

Und noch einmal auf deutsch: Sie haben deine Frage ganz offensichtlich überhaupt nicht verstanden. Sie wissen gar nicht, wovon du sprichst. Sie glauben, es ginge um Verwacklungen und ob der Bildstabilisator selber welche verursachen könne. Außerdem beziehen sie sich auf überkorrigierte sphärische Aberrationen, was eine ganz andere Baustelle ist.

-- Olaf
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I have thought it over and I think VR can  not have such an effect on bokeh. In theory (geometrical) there is a perspective shift due to camera shake (you do not shake with the entrance pupil location as the center of the shake). But the shift of background plane (defocused area) during exposure is neglibly small, too small at all to cause such an effect.
The graphics on [a href=\"http://bokehtests.com/Site/Stabilization_and_Bokeh.html]http://bokehtests.com/Site/Stabilization_and_Bokeh.html[/url] are extremly exaggerated in scale. Keep in mind the sample picture of the deer : 600mm lens; shooting distance was maybe 70 meter (+ background additional 10 metres).
Furhtermore if VR has this shift effect to the defocused areas the effect would cause some kind of motion blur and not "double lines". The effect that was shown here in sample pictures occured with super tele lenses and tele zooms. Most telezooms show some kind of over corrected spherical correction in the tele range and together with the high amount of defocusing  effects (which the human eye does not notice in normal vision) the "bad bokeh" appears. So in the end Nikon answer was OK.
« Last Edit: May 20, 2008, 07:49:32 AM by Mikael » Logged
kshuler
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« Reply #41 on: May 20, 2008, 11:09:06 AM »
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I have thought it over and I think VR can  not have such an effect on bokeh. In theory (geometrical) there is a perspective shift due to camera shake (you do not shake with the entrance pupil location as the center of the shake). But the shift of background plane (defocused area) during exposure is neglibly small, too small at all to cause such an effect.
The graphics on http://bokehtests.com/Site/Stabilization_and_Bokeh.html are extremly exaggerated in scale. Keep in mind the sample picture of the deer : 600mm lens; shooting distance was maybe 70 meter (+ background additional 10 metres).
Furhtermore if VR has this shift effect to the defocused areas the effect would cause some kind of motion blur and not "double lines". The effect that was shown here in sample pictures occured with super tele lenses and tele zooms. Most telezooms show some kind of over corrected spherical correction in the tele range and together with the high amount of defocusing  effects (which the human eye does not notice in normal vision) the "bad bokeh" appears. So in the end Nikon answer was OK.
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Hi Mikael-

As the author of the exaggerated graphics that you referred to, I have to say, you are correct... I massively exaggerated the picture to illustrate the point, not to show EXACTLY what happens in the lens.  Anybody who can't hold their lens steady to the point that it shifts 45 degrees each way is going to have some blurry pictures!  BUt the point is still valid.  Just as you wouldn't shake as much as that, you also would not have the image shifting from the left side of the picture all the way to the right.  BUt the question is, what if there is a 1 degree shake, or a 0.1 degree shake.  How much image shift would there be due to parallax?  I intended to figure this out but then went on to bigger and better things, and have still not made it back there.  ONe thing is sure-- the distance a pixel will travel due to parallax and a 0.1 degree shake will be much more on  600 mm lens than a 24mm lens.

As for your other point, I also agree.  A unidirectional shake will produce a motion blur.  But this will only occur if there is a single direction shake... if you shake from left to right ONLY during a photo.  The minute the lens moves from one direction to the other, you will get a longer exposure for the object right at the point when directions change.  and if you change multiple times, you will get two or more bright lines each time you change directions during your shake.  So it is less important, or completely negligible, if your shutter speed is very fast.  But if your shutter speed is slow enough to involve at least one change in direction you end up with a motion blur and bright spot.  If you have time to change directions in your shake twice, you get the double line effect as I described on my website.  Think of a very fast strobe light doing multiple exposures on a pendulum.... when changing directions at the edges of the trajectory the images will get closer and closer together.  The faster and faster the strobe becomes, the closer you get to a real long exposure, and you will see images bunch up on each other at the edges.  THis is the same as I am talking about, only the LENS is the pendulum.

The effect of VR is just that instead of having a big motion blur evenly throughout the image, the "in focus spot" is stabilized to a certain part of the picture.  The effect described above will be more amplified the further away from the "in focus" spot is.

Now, as I have said many times, I am not a lens designer, a physicist, an optical engineer, or even a professional photographer.  I am just an amateur photographer who finds this stuff very interesting, who has been burned by bad bokeh before and wants to understand it.  So I may be wrong here.  But I doubt it.  Parallax is definitely a REAL phenomenon, and should affect ANY image with ANY lens that is rotating around any point other than the entry pupil, and should also be there with non-rotational movements as well.  The reason it affects BOKEH in image stabilized lenses is that motion blur in the in focus area is stabilized.  BUt again, it depends on how fast you shake, how long the lens focal length is, how long your shutter speed is, etc.  And while I have very little doubt this phenomenon exists, the question is-- is it big enough to be relevant?  Perhaps the shake when translated to the image only accounts for a motion of 0.0002mm at the sensor plane.... well, you won't be seeing this effect then.

If anyone can do the math that would be great.  I can do it, but it will take time because I will have to figure out how to do it.  If anyone knows and could tell me or do the calculation themselves, let me know!

Sorry for yet another very wordy reply.

KLaus
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Mikael
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« Reply #42 on: May 21, 2008, 05:44:04 AM »
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Hi Mikael-

As the author of the exaggerated graphics that you referred to, I have to say, you are correct... I massively exaggerated the picture to illustrate the point, not to show EXACTLY what happens in the lens.  Anybody who can't hold their lens steady to the point that it shifts 45 degrees each way is going to have some blurry pictures!  BUt the point is still valid.  Just as you wouldn't shake as much as that, you also would not have the image shifting from the left side of the picture all the way to the right.  BUt the question is, what if there is a 1 degree shake, or a 0.1 degree shake.  How much image shift would there be due to parallax?  I intended to figure this out but then went on to bigger and better things, and have still not made it back there.  ONe thing is sure-- the distance a pixel will travel due to parallax and a 0.1 degree shake will be much more on  600 mm lens than a 24mm lens.

As for your other point, I also agree.  A unidirectional shake will produce a motion blur.  But this will only occur if there is a single direction shake... if you shake from left to right ONLY during a photo.  The minute the lens moves from one direction to the other, you will get a longer exposure for the object right at the point when directions change.  and if you change multiple times, you will get two or more bright lines each time you change directions during your shake.  So it is less important, or completely negligible, if your shutter speed is very fast.  But if your shutter speed is slow enough to involve at least one change in direction you end up with a motion blur and bright spot.  If you have time to change directions in your shake twice, you get the double line effect as I described on my website.  Think of a very fast strobe light doing multiple exposures on a pendulum.... when changing directions at the edges of the trajectory the images will get closer and closer together.  The faster and faster the strobe becomes, the closer you get to a real long exposure, and you will see images bunch up on each other at the edges.  THis is the same as I am talking about, only the LENS is the pendulum.

The effect of VR is just that instead of having a big motion blur evenly throughout the image, the "in focus spot" is stabilized to a certain part of the picture.  The effect described above will be more amplified the further away from the "in focus" spot is.

Now, as I have said many times, I am not a lens designer, a physicist, an optical engineer, or even a professional photographer.  I am just an amateur photographer who finds this stuff very interesting, who has been burned by bad bokeh before and wants to understand it.  So I may be wrong here.  But I doubt it.  Parallax is definitely a REAL phenomenon, and should affect ANY image with ANY lens that is rotating around any point other than the entry pupil, and should also be there with non-rotational movements as well.  The reason it affects BOKEH in image stabilized lenses is that motion blur in the in focus area is stabilized.  BUt again, it depends on how fast you shake, how long the lens focal length is, how long your shutter speed is, etc.  And while I have very little doubt this phenomenon exists, the question is-- is it big enough to be relevant?  Perhaps the shake when translated to the image only accounts for a motion of 0.0002mm at the sensor plane.... well, you won't be seeing this effect then.

If anyone can do the math that would be great.  I can do it, but it will take time because I will have to figure out how to do it.  If anyone knows and could tell me or do the calculation themselves, let me know!

Sorry for yet another very wordy reply.

KLaus
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Hello Klaus
I did not intend to critisize your graphics as to be exaggerated; that is what they were intended to be, to illustrate the theory of the VR effect.
Mathematically there is an effect, but I think other defocussing effects and lens specific bokeh characteristics are far more predominant. I really tried to reproduce the effect using a 400 mm lens; I took a lot of photos and tried to get as close as possible to the shutter speed where the IS just could compensate the shake (1/60 s) and compared that to the photos taken from tripod. Both were sharp on the plane I focused at and had exactly same bokeh.

So if someone could do the same, maybe we will have proof that the effect exists; I doubt it.
« Last Edit: May 21, 2008, 05:56:41 AM by Mikael » Logged
01af
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« Reply #43 on: May 21, 2008, 03:16:45 PM »
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How much image shift would there be due to parallax? [...] And while I have very little doubt this phenomenon exists, the question is---is it big enough to be relevant?[a href=\"index.php?act=findpost&pid=196797\"][{POST_SNAPBACK}][/a]
Exactly that is the question indeed.

I tried to compute the image shift due to parallax, and I came up with a very complex formula which is hard to compute due to numerical issues, as numbers of vastly different orders of magnitude are involved.

Anyway, my results so far indicate that image shift due to parallax generally is pretty small generally but under certain circumstances can be the same order of magnitude as the pixel pitch of a typical DSLR camera. So it can be up to a few pixels which definitely has the potential to affect bokeh.

For example if you're shooting a subject at a distance of 20 m/65 ft with a 600 mm lens, and if the background is at a distance of 25 m/80 ft, and if your lens revolves by 0.5° during the exposure around a pivot point 10 cm/4" in front of the entrance pupil then the background's image will get shifted by approx. 0.005 mm more than the subject's image. With a 12 MP APS-C camera this corresponds to approx. one pixel. The shift will increase (approx. proportionally) with the distance between subject and background, with the distance between entrance pupil and pivot point of the shake (this can be pretty long with telephoto lenses), and of course with the shake itself; it will decrease with the distance to the subject (i. e. more shift difference at close range).

-- Olaf
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Mikael
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« Reply #44 on: May 24, 2008, 08:00:19 AM »
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Hi Mikael-

As the author of the exaggerated graphics that you referred to, I have to say, you are correct... I massively exaggerated the picture to illustrate the point, not to show EXACTLY what happens in the lens.  Anybody who can't hold their lens steady to the point that it shifts 45 degrees each way is going to have some blurry pictures!  BUt the point is still valid.  Just as you wouldn't shake as much as that, you also would not have the image shifting from the left side of the picture all the way to the right.  BUt the question is, what if there is a 1 degree shake, or a 0.1 degree shake.  How much image shift would there be due to parallax?  I intended to figure this out but then went on to bigger and better things, and have still not made it back there.  ONe thing is sure-- the distance a pixel will travel due to parallax and a 0.1 degree shake will be much more on  600 mm lens than a 24mm lens.

As for your other point, I also agree.  A unidirectional shake will produce a motion blur.  But this will only occur if there is a single direction shake... if you shake from left to right ONLY during a photo.  The minute the lens moves from one direction to the other, you will get a longer exposure for the object right at the point when directions change.  and if you change multiple times, you will get two or more bright lines each time you change directions during your shake.  So it is less important, or completely negligible, if your shutter speed is very fast.  But if your shutter speed is slow enough to involve at least one change in direction you end up with a motion blur and bright spot.  If you have time to change directions in your shake twice, you get the double line effect as I described on my website.  Think of a very fast strobe light doing multiple exposures on a pendulum.... when changing directions at the edges of the trajectory the images will get closer and closer together.  The faster and faster the strobe becomes, the closer you get to a real long exposure, and you will see images bunch up on each other at the edges.  THis is the same as I am talking about, only the LENS is the pendulum.

The effect of VR is just that instead of having a big motion blur evenly throughout the image, the "in focus spot" is stabilized to a certain part of the picture.  The effect described above will be more amplified the further away from the "in focus" spot is.

Now, as I have said many times, I am not a lens designer, a physicist, an optical engineer, or even a professional photographer.  I am just an amateur photographer who finds this stuff very interesting, who has been burned by bad bokeh before and wants to understand it.  So I may be wrong here.  But I doubt it.  Parallax is definitely a REAL phenomenon, and should affect ANY image with ANY lens that is rotating around any point other than the entry pupil, and should also be there with non-rotational movements as well.  The reason it affects BOKEH in image stabilized lenses is that motion blur in the in focus area is stabilized.  BUt again, it depends on how fast you shake, how long the lens focal length is, how long your shutter speed is, etc.  And while I have very little doubt this phenomenon exists, the question is-- is it big enough to be relevant?  Perhaps the shake when translated to the image only accounts for a motion of 0.0002mm at the sensor plane.... well, you won't be seeing this effect then.

If anyone can do the math that would be great.  I can do it, but it will take time because I will have to figure out how to do it.  If anyone knows and could tell me or do the calculation themselves, let me know!

Sorry for yet another very wordy reply.

KLaus
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Maybe on lens based IS systems the shift of the lenses itself has general negative impact on the lens performance, which also cause bokeh to get worse than when turned OFF or when compared to sensor based IS cameras.
« Last Edit: June 02, 2008, 02:03:08 PM by Mikael » Logged
kshuler
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« Reply #45 on: June 27, 2008, 02:17:16 PM »
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For simple geometric reasons, an image stabilizer (no matter whether it's in-lens or in-body) can eliminate camera shake only at the plane of focus. For out-of-focus parts of the image, the same IS action that reduces shake at the plane of focus will amplify shake elsewhere.
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Hi Olaf-

So I have been thinking about this for the last couple hours.... I am starting to change my idea of how this all works.  I figured you seem pretty knowledgeable, so I thought I might pose this here and ask for some help.

My understanding of how IS works was the same as yours- you could only stabilize a single plane of focus, and the rest would not be as affected by image stabilization.  Now I am not so sure this is correct.  If you imagine a rotational motion blur on a non-image stabilized lens (say you took pictures of point sources at 10m, 10m, and 1000m), wouldn't you see a streak of light that would be EXACTLY the same distance for all three points of light?  I think the motion blur will be exactly the same, as long as you are rotating about the entrance pupil (otherwise how does a panoramic tripod work-- you rotate it a bunch of degrees and the foreground and background objects move the same distance on the sensor plane).  My guess is that the IS systems work by assuming that the rotational axis of your shake is AT THE ENTRANCE PUPIL.  This means that the IS system (or SSS) can stabilize the ENTIRE IMAGE at ALL focal planes.  This would be much easier to implement by a lens manufacturer.  Of course, there will be very slight parallax shifts because you will not be rotating precisely around the entrance pupil, but the effect should not be that important, at least not on anything other than a very long focal length lens.

Is this incorrect?  As for this topic, I had previously stated that if this effect exists at all, then it could be explained by parallax error.  It turns out that after thinking about it, IF it truly can only stabilize a single focal plane, then there is a much bigger effect than parallax that would explain the phenomenon that people are reporting here.  I still think the answer to this question is most likely that the changed optics of the IS versions of their lenses just overcorrected spherical aberration and led to a bad case of nisen bokeh.

Let me know what you think.

Klaus
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01af
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« Reply #46 on: June 27, 2008, 06:08:58 PM »
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I think the motion blur will be exactly the same, as long as you are rotating about the entrance pupil ...[a href=\"index.php?act=findpost&pid=204052\"][{POST_SNAPBACK}][/a]
That's right. The problem arises when rotating around some arbitrary pivot point which is not identical to the entrance point. This would introduce some error into the system ... and the point is: the error will vary for the different subject distances. You'd get a different error for every distance.


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My guess is that the IS systems work by assuming that the rotational axis of your shake is AT THE ENTRANCE PUPIL.[a href=\"index.php?act=findpost&pid=204052\"][{POST_SNAPBACK}][/a]
This is what I am thinking, too.

A camera has six degrees of freedom to move in three-dimensional space. There are three axes, and for each axis the camera can do two things: move along and rotate around. Current image stabilizers sense, and compensate for, only two of the six possible movements: rotations around the lateral and the vertical axes (i. e. pitch and yaw). Rotations around one pivot point can be expressed as a linear combination of rotations around some other pivot point plus shift movements along the axes. So neglecting shifts basically is the same as assuming the lens' optical center, i. e. the entrance point (which is where the entrance pupil and the optical axis intersect), as the pivot point indeed.


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This means that the IS system can stabilize the ENTIRE IMAGE at ALL focal planes.[a href=\"index.php?act=findpost&pid=204052\"][{POST_SNAPBACK}][/a]
No, it doesn't. All parts of the image will get stabilized evenly only when the pivot point of the shake actually is at the entrance point.


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Of course, there will be very slight parallax shifts because you will not be rotating precisely around the entrance pupil, but the effect should not be that important, at least not on anything other than a very long focal length lens.[a href=\"index.php?act=findpost&pid=204052\"][{POST_SNAPBACK}][/a]
Well ... in my previous post I explained that, when rotating around a pivot point which is not the entrance point, the difference between the parallaxes for different subject distances can exceed the pixel pitch of a modern DSLR camera and thus, actually might affect bokeh.


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I still think the answer to this question is most likely that the changed optics of the IS versions of their lenses just overcorrected spherical aberration and led to a bad case of nisen bokeh.[a href=\"index.php?act=findpost&pid=204052\"][{POST_SNAPBACK}][/a]
Of course, it is possible that other effects besides parallax errors also may affect bokeh. Optical image stabilizers work by deliberate decentering of the lens---which typically does not exactly improve the optical performance. It's possible that an IS lens in decentered state shows a bokeh that's different from the bokeh in centered state. It also is possible that this phenomenon might affect bokeh to a more significant degree than the parallax errors. The two phenomena also may add up and emphasize each other.

Just speculating ...

Perhaps I should point out that the different bokehs at centered and decentered states (if they actually are different in the first place) is a concern with in-lens image stabilizers only, while the effect of parallax errors concerns all types of image stabilizers, in-lens and in-body.

-- Olaf
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kshuler
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« Reply #47 on: June 29, 2008, 12:39:27 AM »
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hmmmm. Intersting.  I have some stuff to think about and try to understand.  Sorry if this rambles a bit.  Thinking about the way that we both, I think, assume that optical stabilization works (I agree that only 2 degrees of freedom are likely compensated for), while it doesn't TECHNICALLY stabilize the whole image plane, in reality it pretty much does.  If you were to mount a lens on a swivel mount attached directly to its entrance pupil, and do whatever gyrations you might want, the optical stabilization would stabilize the WHOLE image, no?  It would seem then that ALL of the difference between how the in focus object is affected by the stabilization system and how out of focus objects are so affected would be due to PARALLAX.  

If we assume that the IS system presumes rotation about the entrance pupil, there is no need for distance input from the camera, so it attempts to stabilize the whole image.  And, it should be pretty successful at it, I would think.  Since parallax will affect the whole image at EVERY distance (albeit greater for close objects and less for distant objects), this should pretty much stabilize the whole image, plus or minus a few pixels here or there that will be slightly different for close up and background images?  We should not be talking about a single plane of focus that is completely rock stable, and then some areas outside of that plane that widely gyrate... merely a few pixels here or there, right?  Similar to the amount you had calculated previously in one of the above posts.

If this is true, then optical stabilization should work much better (a few pixels less blur) on distant objects than close objects.  Moreover, this may not only affect the bokeh of a shot, but if this is all correct, it may actually affect the sharpness of the image, and provide a maximum allowable sharpness for image stabilized lenses that will be different depending on how far from the camera the object is.  Again, the effect should be the same if it is SSS, or IS/VR.

Also, one thing I wondered about that you had said above.  You mentioned that the centering of the lens changing should only affect IS/VR type lenses and not sensor shift technologies.  There is something different, however, that can affect sensor shift technologies, as well, and it is pretty much analagous, I would think.  When the sensor shifts, the light path through the lens that is hitting a particular sensor pixel will be DIFFERENT than it was before the shift.  Since different areas of lenses may have different bokeh characteristics, this should be able to affect bokeh too (an example would be the poor bokeh performance of the sony Zeiss 24-70 SSM lens that seems to have really quite acceptable bokeh when focused at 10 feet in the center, but extremely poor bokeh at the same distance on the sides of the lens).  If you don't believe that lenses have different bokeh in different areas of the lens, see my tests of the Zeiss mentioned above at http://www.bokehtests.com/Site/Sony_Zeiss_24-70_Bokeh.html.

Thanks for the time and discussion.  I am learning a lot.

Klaus
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01af
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« Reply #48 on: June 29, 2008, 09:40:07 AM »
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If you were to mount a lens on a swivel mount attached directly to its entrance pupil, and do whatever gyrations you might want, the optical stabilization would stabilize the WHOLE image, no?[a href=\"index.php?act=findpost&pid=204282\"][{POST_SNAPBACK}][/a]
I think it would indeed, as long as you don't exceed the stabilizer's working range. However hand-held camera shake does not generally pivot around the entrance point exactly.


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It would seem then that ALL of the difference between how the in-focus object is affected by the stabilization system and how out-of-focus objects are so affected would be due to PARALLAX.[a href=\"index.php?act=findpost&pid=204282\"][{POST_SNAPBACK}][/a]
That's the question. I can hardly think of any other effects (besides varying lens bokeh) ... but that doesn't mean too much as I am no expert. I am only rambling, just like you do.


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If we assume that the IS system presumes rotation about the entrance pupil, there is no need for distance input from the camera, so it attempts to stabilize the whole image.[a href=\"index.php?act=findpost&pid=204282\"][{POST_SNAPBACK}][/a]
Good point. However as far as I know, image stabilizers do take distance information into consideration ... so I am a bit confused.

Distance information is good for two things. First: Geometry of the imaging. Second: Properties of the lens. Lenses do change their properties according to focus distance, namely effective focal length (and a few other parameters). Question is: Does the image stabilizer know which lens it is stabilizing? Well, in-lens stabilizers most likely do  . Do in-body systems adapt to the particular lens model? Another question is: Does the stabilizer adapt to the focus distance as such? If so then there is something going on you and me are not aware of.


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We should not be talking about a single plane of focus that is completely rock-stable, and then some areas outside of that plane that widely gyrate ... merely a few pixels here or there, right?[a href=\"index.php?act=findpost&pid=204282\"][{POST_SNAPBACK}][/a]
Right. In most cases, the difference between stabilization of in-focus and out-of-focus parts of the image are minuscule; in some cases they may affect the bokeh to a perceivable degree.


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If this is true, then optical stabilization should work much better (a few pixels less blur) on distant objects than close objects.[a href=\"index.php?act=findpost&pid=204282\"][{POST_SNAPBACK}][/a]
Actually, it does! Image stabilizers don't work well in the macro range. The main reason for this is the fact that the effects of the camera movement's other (uncompensated) four degrees of freedom become more significant at close range.


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There is something different, however, that can affect sensor shift technologies, as well, and it is pretty much analagous, I would think.  When the sensor shifts, the light path through the lens that is hitting a particular sensor pixel will be DIFFERENT than it was before the shift.[a href=\"index.php?act=findpost&pid=204282\"][{POST_SNAPBACK}][/a]
Yes, but still it was there in the image (albeit elsewhere). So it's not really analoguous I'd think.


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... (an example would be the poor bokeh performance of the Sony SAL 24-70 ZA SSM lens that seems to have really quite acceptable bokeh when focused at 10 feet in the center, but extremely poor bokeh at the same distance on the sides of the lens).[a href=\"index.php?act=findpost&pid=204282\"][{POST_SNAPBACK}][/a]
This is true only at full aperture. Most lenses have not-so-good or even poor bokeh at their full apertures. Stop the lens down by one f-stop or even just half an f-stop, and in most cases bokeh will improve considerably.

-- Olaf
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« Reply #49 on: June 29, 2008, 06:53:36 PM »
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This is true only at full aperture. Most lenses have not-so-good or even poor bokeh at their full apertures. Stop the lens down by one f-stop or even just half an f-stop, and in most cases bokeh will improve considerably.

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I would think that sensor shift technologies must also know to some extent what lens they are stabilizing... after all, the distance to shift the sensor would be very different for a 12 mm lens that moves 1 degree and a 1200 mm lens that moves one degree.

As for most lenses having better bokeh at a few stops past wide open aperture, I have read that from several sources, so it must have a solid basis.  My testing of the Sony SAL 24-70 lens, however, does not show that so much, or rather, the story is much more complicated.  It loks like the bokeh produced at the edge of the picture DOES improve, but the bokeh in the center of the lens actually gets slightly worse when going from f/2.8 to f/4 or even f/5.6.  So it seems to be very lens dependent whether Bokeh gets better or worse when stopping down.  but so far that is the only lens I have put through any kind of rigorous testing... but I wouldn't be surprised to see things like this in multiple lenses from any lens line.

Technically, also, if a lens has "good bokeh," that is, specular highlights having a less than sharp border and no ring artifact, one would expect the lens to have WORSE bokeh stopping down.  Since spherical aberration takes place at the edge of the lens, cutting the edge off with an aperture blade will tend to make it more like an "ideal" lens with a complete lack of spherical aberration... in other words, bokeh will get more neutral.  For lenses that are overcorrected for spherical aberration, stopping down should eliminate some of the ring artifact, and thus produce better bokeh, but for lenses with good bokeh stopping down should produce worse bokeh.  Obviously, this is a large generalization, however.  If my understading is wrong, I would love to be corrected!

Klaus
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[a href=\"http://www.bokehtests.com]http://www.bokehtests.com[/url]
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01af
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« Reply #50 on: June 30, 2008, 05:33:05 AM »
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I would think that sensor shift technologies must also know to some extent what lens they are stabilizing ... after all, the distance to shift the sensor would be very different for a 12 mm lens that moves one degree and a 1200 mm lens that moves one degree.[a href=\"index.php?act=findpost&pid=204399\"][{POST_SNAPBACK}][/a]
Yes, sure. I am afraid you didn't get the point here. Of course it's vital to the stabilizer to know the lens' focal length. But does it also know the difference between, say, a tele zoom lens with front-part focusing, set to 200 mm, and a 200 mm prime telephoto lens with internal focusing? After all, both lenses' effective focal lengths will deviate from the nominal 200 mm at focus distances shorter than infinity in different ways, due to the different focusing mechanisms.


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As for most lenses having better bokeh at a few stops past wide open aperture, I have read that from several sources, so it must have a solid basis.[a href=\"index.php?act=findpost&pid=204399\"][{POST_SNAPBACK}][/a]
It does have a solid basis for sure. Of course, it doesn't mean all lenses' bokehs must improve when stopping down. There are exceptions to every rule ... some lenses have bad bokeh at all apertures; some have perfect bokeh wide open. But most lenses exhibit their best bokeh half or one f-stop down. Of course, 'best' for one specific lens does not necessarily mean 'good' by absolute standards.


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It looks like the bokeh produced at the edge of the picture DOES improve, but the bokeh in the center of the lens actually gets slightly worse when going from f/2.8 to f/4 or even f/5.6.[a href=\"index.php?act=findpost&pid=204399\"][{POST_SNAPBACK}][/a]
Umm ... are you sure you're not confusing bokeh and blur here?


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So it seems to be very lens-dependent whether bokeh gets better or worse when stopping down.[a href=\"index.php?act=findpost&pid=204399\"][{POST_SNAPBACK}][/a]
As I just said, in a few rare cases bokeh may not improve with some lenses when stopping down ... but usually it does, and so far I never stumbled across a case where bokeh actually changes from good to bad when stopping down.


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Technically, also, if a lens has "good bokeh," that is, specular highlights having a less than sharp border and no ring artifact, one would expect the lens to have WORSE bokeh stopping down.[a href=\"index.php?act=findpost&pid=204399\"][{POST_SNAPBACK}][/a]
In principle, that's right---the better the bokeh is at full aperture, the more likely it will get worse on stopping down. However I'd say this actually applies to only a handful of lenses in the world.


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Since spherical aberration takes place at the edge of the lens, cutting the edge off with an aperture blade will tend to make it more like an "ideal" lens with a complete lack of spherical aberration... in other words, bokeh will get more neutral.  For lenses that are overcorrected for spherical aberration, stopping down should eliminate some of the ring artifact, and thus produce better bokeh, but for lenses with good bokeh stopping down should produce worse bokeh.  Obviously, this is a large generalization, however.  If my understading is wrong, I would love to be corrected![a href=\"index.php?act=findpost&pid=204399\"][{POST_SNAPBACK}][/a]
Your understanding isn't wrong but it's oversimplifying. There's more to bokeh than just the shape and gradient of the circles of confusion. Even if a 'perfect' blur disk gets its edges cut off, bokeh won't necessarily get worse because other bokeh-affecting parameters may improve. And on the other hand, I have seen images where the blur disks of specular highlights clearly had strong ring artifacts and still the overall impression of bokeh was fairly nice.

-- Olaf
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