12-24mm F4.5-5.6 EX DG ASPHERICAL HSM

[piiooo]

ok so tell me. how does changing the body change the lens? I mean what would the difference if I had a 5D mk.II verses say a 50D?

On a full frame body (eg 5DMkII) this lens is truly 12-24mm 4.5-5.6


On a APS-C body(eg 50D) this lens will be roughly equivalent to 19-38mm 7.1-9.0 (1.6 x longer and 1 1/3 stop slower) , so it will give you still wide but significantly narrower angle, and greater depth of field.