[Forseti]

Towards the bottom of his reply Paul say's - "....for example, a 50mm lens on a 500D or 50D is equivalent to an 80mm Lens on a full-frame D-SLR"

Oh dear, if the so-called experts can't get it right it's little wonder that questions of this nature continue to pop up on the forum. Paul's' reply would, to those that don't know, imply an element of magnification is involved whereas the simple fact is, is that a 50mm lens (or an 80mm lens) will still be a 50mm lens (or 80mm lens) whatever camera it is used on - whether on a crop frame camera or a full-frame camera. In short, the physical characteristics of a lens cannot be changed but the effects are variable depending on the size of sensor.

Might I boldly suggest that the correct answer should have been along the lines of - when setting a focal length of 50mm on a crop frame camera this would give the same field of view (FOV) as if the same lens had been used on a full-frame camera but with a focal length of 80mm having been set. Magnification is not changed - only FOV.

In Paul's reply no mention is made of FOV and this is a glaring omission imho.