IS and image quality

Started Mar 27, 2013 | Questions thread
William Woodruff
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Re: IS and image quality
In reply to bhollis, Mar 29, 2013

If I understand OP's question, he has gotten a lot of really good answers to questions he didn't ask.  My understanding of the question (which tracks with bhollis' response) is whether the absolute imaging quality of a lens is compromised by adding optical IS.  That is different from asking whether, under field conditions, you will get better results with IS.

If I understand the question correctly, the simple answer has to be that optical IS always compromises image quality.  (Again, the practical application of that image quality is a different issue.)  It seems fair to say that a well-designed IS has very little negative impact (considering, for example the 70-200/2.8 II, 70-200/4, 17-55, etc.) but it is inconceivable to me that there is not some degradation.  First of all, every time you pass an image through an additional piece of glass, there will be some light absorption, which is never a good thing.  Furthermore, there is no perfect lens element, the flaws may be small, but they are there, so adding elements adds errors; thus, it seems that addition elements that are in the image management system for some reason other than correcting (as opposed to stabilizing) an image must be detrimental to absolute quality.  And last, it is equally inconceivable to me that an unstable lens element could be completely and consistently error-free in its alignment; and the IS group must, by design, be unstable to do its job.

So my answer, which is not based on testing, but rather is based my best attempt at a logical interpretation of what is possible, is that anything lens designers can do with IS, they can do better without IS.  But not much better.  And that's why most of us really like IS (at least on normal or longer lenses).  Even if I am correct that IS always compromises image quality, that compromise is very, very small with the newest systems.  On the other hand, the practical benefit of having IS can be very significant.  So in the end, IS gives us a very big benefit for a very small detriment.  And that makes it look like a pretty good idea.

So I would love to see the Canon folks use all of their very latest design ability and build a non-IS 70-200/2.8 II without IS just so we could compare them.  But I wouldn't buy it, I would buy the IS version because I don't shoot theoretical images, I shoot real images.

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