WOW! Still discussing on this... we must be very bore while waiting for new Rumors... ha ha ha
I'm also very bore while waiting for "news" on 70D or/and 7D2...
Anyway, the OP questions about IS consist of 2 parts:
1) Does the IS action affect IQ?
I agree with what Mr Bean mention below.
My feeling is that IS would affect IQ as the lens or lenses in the IS unit are moved off center, to negate movement of the photographer. This process of moving the lenses off axis in the optical path would have been "allowed" in the design, but it'll be a case of "....the lesser of 2 evils".
2) Does the extra elements in IS affect IQ?
This questions also consist of 2 parts:
a) Does IS increase the elements in a lens?
This is a very difficult question to answer since I'm not an optics designer, but base on the trend in the lens specification:
Lens with IS had more elements than similar lens without IS. For example:
- EF 70-200mm f/2.8L IS USM has 23 elements, but EF 70-200mm f/2.8L USM only had 18 elements.
- EF 70-200mm f/4L IS USM has 20 elements, but EF 70-200mm f/4L USM only had 16 elements.
b) If IS does require more element, does the extra element affect IQ?
From the other thread, "Can a UV filter affect IQ (sharpness) on a lens?", most of poster agree that a filter does decrease IQ to a certain degree, but the decrease in IQ is negligible if the filter is a very good one. So if adding a filter does decrease the IQ, so does every element in the lens since a filter is also consider an optical element (even air in lens is also consider as an optical element).
By the way, IMHO, using modern coating and manufacturing technology, the decrease in IQ cause by a few extra elements is very minimum, but you gain 3 to 4 stop of advantage with IS, so it's great to have IS as long as it's does not increase the price tag by a lot... ha ha ha
Have a nice day.
PS: Where my 70D or/and 7D2 rumors!!