Quote Originally Posted by ChadS View Post
I've always wondered about that myself. An EF-S lens is too close to the sensor to flip up a mirror if it's a simple hinge. However, if there's any sort of linkage in there it's quite possible that the mirror could clear. Imagine that the back/top of the mirror translated forward and the bottom/front of the mirror swung up at the same time. That would prevent smashing the close-in lens. It's an expensive (and probably loud) solution so there may be a more elegant one than that - but there are solutions.
Contrary to popular belief (and contrary to Wikipedia), the -s in EF-s refers to small image circle. Canon's EF-S have a smaller image circle *and* a short back focus. Other manufacturers' APS-C-only lenses have only a smaller image circle. In fact, 3rd party Canon mount lenses (Sigma 8-16, Tokina 11-16) can be mounted on Canon APS-H/FF bodies with no risk (but severe vignetting, obviously).