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?
Yep, that was me, and wickidwombat's explanation was spot on. Because the crop factor affects angle of view, for a given focal length you need to be further away to get the same framing on an APS-C camera, and that extra distance means a shallower DoF - by a factor of 1.6x (=1.3 stops).
A shallow depth of field is a function of a few factors:
3) The distance between the background and the subject
On Crops as apposed to FF:
The ACTUAL focal length of the lens is MUCH smaller to achieve the equivalent perspective of a FF.
The crop factor on the other hand has NOTHING to do with EXPOSURE
These statements have a few inaccuracies. Subject to background distance affects the OOF blur, but not the depth of field.
Perspective is solely a function of camera to subject distance. You mean equivalent field of view.
It depends on how you define exposure. If you mean shutter speed and f/number, it's true that the crop factor has no effect. But if you mean the exposure triangle, which includes ISO, the FF sensor enables you to bump the ISO up by 1.3 stops and keep the same noise in your image.
You COULD have trouble nailing sharp focus - as since you are shooting wide open - and AF needs good contrast to nail focus - since you are further away - the persons eyes for example are further away etc. so the AF might not be able to nail tack on.
Makes no sense. As Meh states, from a greater distance with the same focal length, DoF is deeper, meaning more of the subject should be in sharp focus. If the framing is the same, the extra distance doesn't matter to the camera's AF system, the subject will cover the same portion of the AF sensor. If you're even further away, even less problem. The place focus errors manifest is with close subjects and long focal lengths, where DoF is thinnest.