April 20, 2014, 12:06:55 AM

Author Topic: Does IS make bokeh busier/less smooth?  (Read 3198 times)

SwissBear

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Re: Does IS make bokeh busier/less smooth?
« Reply #15 on: July 09, 2013, 09:27:37 PM »
I threw the question out there because I thought it's been asked before and I thought there would be a ready answer.

Indeed it has already been asked, but then the discussion came to no ending and definitively not to an easy answer.
But I can offer you the following easy theoretical optical experiment:

Be the camera shake along an axis that does not contain the nodal point of the setup or is orthogonal to the optical axis, for the ease of the argument a up-down movement. At the time of the release, the IS element corrects a the extreme on one side, at shutter close on the other extreme. This results in a sharp subject.
Now be there a main subject 1m away from the sensor and a DOF of 10cm (form 0.95m to 1.05m). Be there two other subjects at 0.9m and 1.1m. Be All subjects static. Now it is easy to imagine what happens during the release (with the described shake). The main subject stays in focus (thanks to the IS), but the relative positions of the two other subjects as seen from the sensor change, or simply, a parallax error occurs.
This results in harsh/unruly/nasty/... bokeh. Sadly, I have no example as I dump such occurings as soon as i spot them.

Conclusion:
Yes, an active IS can have a negative effect on bokeh.
Switching IS off renders always the bokeh that is "normal" for the lens.
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Re: Does IS make bokeh busier/less smooth?
« Reply #15 on: July 09, 2013, 09:27:37 PM »

sunnyVan

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Re: Does IS make bokeh busier/less smooth?
« Reply #16 on: July 09, 2013, 09:43:51 PM »
I threw the question out there because I thought it's been asked before and I thought there would be a ready answer.

Indeed it has already been asked, but then the discussion came to no ending and definitively not to an easy answer.
But I can offer you the following easy theoretical optical experiment:

Be the camera shake along an axis that does not contain the nodal point of the setup or is orthogonal to the optical axis, for the ease of the argument a up-down movement. At the time of the release, the IS element corrects a the extreme on one side, at shutter close on the other extreme. This results in a sharp subject.
Now be there a main subject 1m away from the sensor and a DOF of 10cm (form 0.95m to 1.05m). Be there two other subjects at 0.9m and 1.1m. Be All subjects static. Now it is easy to imagine what happens during the release (with the described shake). The main subject stays in focus (thanks to the IS), but the relative positions of the two other subjects as seen from the sensor change, or simply, a parallax error occurs.
This results in harsh/unruly/nasty/... bokeh. Sadly, I have no example as I dump such occurings as soon as i spot them.

Conclusion:
Yes, an active IS can have a negative effect on bokeh.
Switching IS off renders always the bokeh that is "normal" for the lens.

I think I got what the gist of what you were saying. It seems to make sense.  I need to do some experiments. In the meantime it doesn't hurt to turn off IS. I almost always use 1/125 or higher shutter speed anyway. With my 6D I don't really hesitate to increase ISO.



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bdunbar79

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Re: Does IS make bokeh busier/less smooth?
« Reply #17 on: July 10, 2013, 08:39:29 PM »
I don't think that makes any sense at all.  I could say, my unsteady hands affect bokeh.  Sure they do, but it's not my actual hands doing it. 

The real answer is no, the IS being on itself, objectivley, doesn't affect bokeh one bit, vs. being off, all else equal, no shake, nothing unusual.

sunnyVan

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Re: Does IS make bokeh busier/less smooth?
« Reply #18 on: July 10, 2013, 09:51:00 PM »
The verdict so far seems split with some saying yes and some saying no.

It would be nice to have more inputs. But it seems like people are busy bashing this guy who tries to leave canon for Nikon's "superior" sensor.

Anyway, I'll do some testing when I get a chance.
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alexanderferdinand

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Re: Does IS make bokeh busier/less smooth?
« Reply #19 on: July 10, 2013, 10:10:58 PM »
The argument with the axis: why is the object in focus not affected, but the not in focus are?
Wouldnt this change the axis on the in focus object too?

SwissBear

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Re: Does IS make bokeh busier/less smooth?
« Reply #20 on: July 11, 2013, 05:03:52 AM »
The argument with the axis: why is the object in focus not affected, but the not in focus are?
Wouldnt this change the axis on the in focus object too?

Well, thats the magic of the IS ;)

The angle of the plane of focus in relation to the object/camera may be affected, that depends on the way the IS works. If the IS works as a shift element, then the focal plane is not affected. If it works as a tilt element, then there is a minimal change:
If the maximal correctable shift is some 1mm in any direction (my guess, maybe its lower), the subject distance is still 1m, then the angular change of the focal plane is:
0.001/1=tan(alpha), solve for 2alpha, which is approx. 0.002rad or 0.115 degree.
I have no clue how an IS really works...

But more important is the actual position shift of the objects out of focus:
The position change of the far object can also be calculated: 2mm/1000m=x/10mm => x=0,2mm (32 pixel shift on a 5DIII, or some 46 pixel on the 18MP crop sensor as found in the 600D and others)
These are purely theoretical values, as the IS element is surely near the nodal point to lessen this parallax error.

I try to think of CR the next time I get a bad pic due to IS interference.
If you want to force bad bokeh, try the 70-200 or the 24-105 at its maximum focal length, focus on something like a single reed, a flower/fruit in a tree - something that is isolated, small so you get near MFD and has anyway a wild background. At least all pics i got with this IS-bad-bokeh were on such a setup - normally they were visible on the camera LCD so I took it again.
« Last Edit: July 11, 2013, 05:43:15 AM by SwissBear »
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Re: Does IS make bokeh busier/less smooth?
« Reply #20 on: July 11, 2013, 05:03:52 AM »