I have a very hard time believing that you can adjust a zoom lens by changing the back focus distance by 5 mm or whatever. Lens focus tends to change a little when zooming and a little when stopping down the aperture. So a fixed number probably isn't always going to work.
Good point, actually. In canonrumors and dpreview forums, people are saying that AF microadjust makes sense only at a specific focal length and subject distance. So, if I understand that correctly, you would need to microadjust continuously if your lens isn't properly built. Also, I read that, supposedly, Canon cameras can store one microadjustment per lens, so that at least you don't have to re-adjust every time you swap lenses. However, that kind of suggests that Canon is thinking that on microadjustment per lens is enough, which contradicts the F / subjecy distance adjustment thing.
A bit confusing.
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