Canon CEO: Semi-Pro APSC DSLRs are following the Dodo ...

For primes anything from roughly 35/2 covers the FF sensor even if it's designed only for APS-C.  There might be different compromises in a crop design and a FF design so edge performance might suffer (or it might not).

Where the difference shows is (a) in lenses wider than about 35mm and (b) in zooms.  This example (by my brother on his D700) shows how the Sigma 10-20/4-5.6 vignettes at 14mm.  The outer rectangle is the full FF frame; the inner one is trying to show a 92% VF.

--
---
Gerry
_______________________________________
First camera 1953, first Pentax 1985, first DSLR 2006
http://www.pbase.com/gerrywinterbourne