IS and IQ..

[andnowimbroke]

Yours is the good company. Others choose the "beg forgiveness rather than ask permission" approach. I would hope Canon is more like yours. Sometimes I question though...

@daniel
Good points about room available.

[Dr Croubie]

I'm sure canon wouldn't mind me stealing images from their Camera Museum for this discussion, so here's some block diagrams:


70-200 f/2.8L non-IS


70-200 f/2.8L IS mk1


70-200 f/2.8L IS II


70-200 f/4L non-IS


70-200 f/4L IS


Now, I'm not a lens designer, but still, here's my thoughts:
- Going from 2.8 to 2.8 IS, they've just added in the 'triplet' that moves during stabilising, and changed an element or two on either side to direct the light rays through it properly. Also note that they've "bent" the rays more towards the centre of the lens, because all the light rays have to go through the small IS-group, then they have to bend them back out (that's how a telephoto works).

- In the non-IS version, where there's a big gap where there's no IS group, the light rays can go through unimpeded, and with less bending.

- Going from the IS mk1 to IS mk2, they've put the Fluorite element right before the IS group. The light rays still get bent a lot to go through the small IS-group elements (red square), but they're bent through the fluorite. Fluorite elements bend light a lot more, and a lot more cleanly than normal glass (but it costs a packet). They've also brought in 2 more UD elements (between Fluorite and normal glass in bending quality) at the front, where the light gets bent the most (the 'telephoto' design). That's probably accounting for a lot of the extra quality in the IS mk2 from the IS mk1.

- In the f/4 lenses, they're pretty much the same design, and they're also not that dissimilar in IQ, they've moved a UD lens around to bend the light better to go through the IS unit, but besides that they're practically the same lens.

- In general though, more elements in a lens makes for a 'darker' lens (and can introduce some problems), but the more elements the more it is corrected for aberrations, distortion, CA, field curvature, whatever (look at what you get out of a lensbaby, that's what a single or double-element lens looks like with no aberration correction...). So the IS unit in some lenses is helping out the IQ, in some cases it's actually making it worse. In general though, the newer and more expensive lenses (especially the case with IS) are going to outperform the older ones...