It is my understanding that the S lens focal lengths are rated the same as full frame 35mm focal lengths and that the reason they are only for use on the smaller size sensors is a mechanical thing at thje mount.....
That's an interesting and correct point on the EF-S lens on an EF-S body like the 20D, Digital Rebel etc., although this thread started with 1D series cameras with full frame EF mount.
The lens mount of the EF-S lenses is mechanically keyed differently than the EF lenses to prevent it from being used with the EF mount bodies, not sure the exact reason but I seem to recall is due to mechanical interference, the benifit is Canon can now make a smaller, more compact DSLR with matching set of smaller lenses to keep the cost down and stay competitive, i.e. a smaller f2.8 lens is cheaper to make than a larger f2.8 lens.
In a pinch last year when I didn't have all my own lenses with me, I did try to use an EF-S lens from a Digital Rebel our company bought on my EF body and it just plain didn't match.
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