And I'd better clarify my 'shorter at the back' bit which on re-reading looks a bit 'wrong!' I meant to say a bit 'closer' to the sensor than a conventional lens so they would foul-up the mirror if you fit them to an incompatible camera.
Most telephoto lenses where the rear element is a fair way inside the lens body seem to be OK, giving a near-telecentric design as described by Leif, so it seems as if the biggest advantage is in the re-designing of shorter focal lengths.
I've got one of Canon's basic 28-80mm zooms which gives reasonable results on my EOS 50E body but the images on my 20D body are not very good other than 'snapshot' standard.
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