# Does IS make bokeh busier/less smooth?



## sunnyVan (Jul 9, 2013)

I realize it's unfair to compare the bokeh quality between 135L and 70-200 f4. Not until I got my 135L did I realize how busy the bokeh my tele zoom produces. My question is whether it is due to the use of IS. I've heard someone mention that turning off IS on 70-200 2.8 would make the bokeh smoother so I would wonder about the same on my f4 IS. My preliminary conclusion is that the bokeh quality is the whole point of having a prime and it is irreplaceable by a tele zoom.


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## Pi (Jul 9, 2013)

It is mainly due to a different lens design. IS can change the image slightly but it cannot make your 70-200 like the 135, even both at f/4. Try it.


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## Admin US West (Jul 9, 2013)

Bokeh is not affected by IS, it is determined by the lens design. Typically, prime lenses will be better than zooms.


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## sunnyVan (Jul 9, 2013)

Pi said:


> It is mainly due to a different lens design. IS can change the image slightly but it cannot make your 70-200 like the 135, even both at f/4. Try it.



I'm not trying to make my tele produce the same bokeh as the 135L. If I could, there would be no place for the 135L in my bag. I'm trying to see if turning off IS would make the bokeh improve somewhat on my zoom. And if IS does have an impact on bokeh quality (a hypothesis), why bother having it if I consistently use at least 1/125 shutter speed? And people say they want a new 135L with IS, if the hypothesis stands, wouldn't that mean the bokeh quality won't be the same as the original 135L?


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## sunnyVan (Jul 9, 2013)

CR Backup Admin said:


> Bokeh is not affected by IS, it is determined by the lens design. Typically, prime lenses will be better than zooms.



I see. That's what I thought. Thanks.


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## Random Orbits (Jul 9, 2013)

I've seen some nasty "bokeh" out of the 70-200 f/2.8 IS II and 100L, but that is usually when the shutter speed is slow, so I do think that IS can make it worse under certain conditions. The IS can only compensate at the targeted focal length, so objects at other distances may be moving more with IS than without it. Good technique/keeping the camera/lens as still as possible seems to help because IS is not as active. Longer exposures with wind-blown trees lead to a similar effect. I don't recall encountering bad bokeh in good light (i.e. less than 1/1000s)


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## bchernicoff (Jul 9, 2013)

I think a good question in general is whether there is any change in IQ when the IS elements are shifted to compensate for motion.


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## sunnyVan (Jul 9, 2013)

bchernicoff said:


> I think a good question in general is whether there is any change in IQ when the IS elements are shifted to compensate for motion.



It's a good question but different from the original one I asked. In my case the primary subject looks perfectly sharp with IS on. The background under some circumstances would look a little nervous, a little busy. 

If the IS helps make the subject sharp but I have to sacrifice the smoothness of the background, I may want to turn IS off and use it very sparingly. If IS doesn't really do anything to the background I could leave it on at all times. Ever since I have owned my tele I have never turned IS off.


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## Rienzphotoz (Jul 9, 2013)

Hmmm ... that's the first time I ever heard someone question the quality of bokeh with/without IS ... I'll have to test and see but I don't think IS has any bearing on bokeh.


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## Knut Skywalker (Jul 9, 2013)

The lens design with IS is a different, there is more glass and air the light has to travel through as with a prime so maybe that is a factor.


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## alexanderferdinand (Jul 9, 2013)

Talking about nasty bokeh:
The nastiest my equipment has to offer is the 70-200/2,8II with the TC 1,4.
Useful combination for outdoor sports, very good IQ in focus, but.....

No, IS does not have an effect on the bokeh.


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## Dantana (Jul 9, 2013)

True bokeh is the result of things being out of focus and is affected by lens design, aperture blade design, etc.

IS addresses blur caused by motion of the camera. It doesn't have anything to do with focus directly.

Not saying you're not seeing what you're seeing, just that it might be a case of the blur rather than the bokeh.


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## Pi (Jul 9, 2013)

sunnyVan said:


> I'm not trying to make my tele produce the same bokeh as the 135L. If I could, there would be no place for the 135L in my bag. I'm trying to see if turning off IS would make the bokeh improve somewhat on my zoom. And if IS does have an impact on bokeh quality (a hypothesis), why bother having it if I consistently use at least 1/125 shutter speed? And people say they want a new 135L with IS, if the hypothesis stands, wouldn't that mean the bokeh quality won't be the same as the original 135L?



Well, try it, and let us know. I do not expect too much difference. On the other hand, a lens having IS would have a different design, and this _might_ affect the bokeh visibly. 

IMO, the main reason for not so great bokeh with some lenses is because they were optimized for sharpness. This includes some of the 70-200 lenses, the Sigma 35, and the 18-35, and maybe others. There is a well known and easy to google Zeiss document about bokeh and it explains how over- or under- corrected aberrations affect bokeh.


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## sunnyVan (Jul 9, 2013)

Pi said:


> sunnyVan said:
> 
> 
> > I'm not trying to make my tele produce the same bokeh as the 135L. If I could, there would be no place for the 135L in my bag. I'm trying to see if turning off IS would make the bokeh improve somewhat on my zoom. And if IS does have an impact on bokeh quality (a hypothesis), why bother having it if I consistently use at least 1/125 shutter speed? And people say they want a new 135L with IS, if the hypothesis stands, wouldn't that mean the bokeh quality won't be the same as the original 135L?
> ...



Thanks for your input. I will definitely try. I don't expect very much difference either but for the sake of argument I'm going to try without IS for a week and see what I get. I'm open to experimentation. 

I threw the question out there because I thought it's been asked before and I thought there would be a ready answer.


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## Pi (Jul 10, 2013)

sunnyVan said:


> I threw the question out there because I thought it's been asked before and I thought there would be a ready answer.



It is certainly and interesting question, especially whether IS would alter the design in a way that would change the bokeh visibly. Unfortunately, we will never now, since the battle now is to beat the competition when shooting charts. Every new version of a lens which does not have higher MTF numbers would be declared a failure by the Internet. And you can't argue with the Internet(s).


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## SwissBear (Jul 10, 2013)

sunnyVan said:


> 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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## sunnyVan (Jul 10, 2013)

SwissBear said:


> sunnyVan said:
> 
> 
> > I threw the question out there because I thought it's been asked before and I thought there would be a ready answer.
> ...



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 (Jul 11, 2013)

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.


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## sunnyVan (Jul 11, 2013)

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 (Jul 11, 2013)

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?


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## SwissBear (Jul 11, 2013)

alexanderferdinand said:


> 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.


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