Well.... This is one:
So...just as the 'crop factor' affects the effective focal length, it also affects the DoF - by the same 1.6x. That means that for the same framing you'd get on FF, an f/4 lens on APS-C is equivalent to f/6.4 on FF. That means very poor background blur unless you are very close to your subject and the background is well-separated. Not that it can't be done...it's just not ideal.
If I am understanding it right, F4 on a Full Frame is about F6.4 on a APS-C? So, to truly get down to F2.8, I should go FF? And the other part was, the EF-s 17-55 F2.8, is it like F4 on an APS-c, or is it trult F2.8?
Maybe I need to get to a website that explains this better.....