Ok, keeping in mind that my brain has trouble understanding a lot of the technical things associated with DSLR's, 35mm ones were easier,
currently playing with a 450D and wishing soon to go up to a either 40/50D or 5D most likely 2nd hand preferably off here rather then fleabay,
my question, I tend to shoot a lot of friends at events in costumes etc and since playing with the 50mm nifty fifty lens and realising the f1.8/2.8 range is perfect and lets in loads more light, I think a zoom lens would be next on the list, lighting is usually ok maybe a touch of fill flash, but the extra light of a 2.8 should be enough.
Currently looking at the Tamron 17-55mm f2.8 (pretty sure thats its specs) but can this lens be used on the 40d/5d camera bodies as they go from apc (small frame?) to 5d full frame?
Are all lens interchangeable with the different bodies? ie 450d - 40d- 5d?
Or have I been reading information incorrectly? Sorry if this questions been addressed earlier i couldnt find it.

Thanks in advance

Grem