The OP posted in another thread http://www.canonrumors.com/forum/index.php?action=profile;area=showposts;u=375238
"What does AFMA mean??? "
People join the forum to learn, so it is great that he has asked. But, it's somewhat tough on B&H when people buy a camera from them and return it because they haven't read the manual.
About a 300mm on FF or 480mm on crop, indeed a 300mm stays a 300mm from way of compression of the picture. However when I look to the pixels covered by an object then the 300mm on a crop will give the same result as a 480mm on a FF. Not on a base of compression, but only on pixels on the subject. Also the dof will be much smaller on the crop compared to a FF, with the same lens. For that reason, AFMA for a 300mm on a crop is very important.
As AlanF pointed out, I wasn't, and never have, denigrated crop cameras, indeed I own one. But people, even now, seem strangely uncomprehending of what they actually do, I am sure in large part to the initial sales push by marketing departments and then continued lack of real education for the majority of buyers who, in truth, probably don't actually care too much.
I also agree with AlanF on how tough this purchasing of complex equipment is on retailers when purchasers won't even read the manual. Though manuals have taken a strange persona of their own nowadays, computers come without one and my hotshoe flash manual is several hundred pages! But declaring issues and problems without a fairly good understanding of the problem is the new way of life it seems. That is cool, it means there is even more work for those that can deliver........
As for 300mm and crop camera dof, as always, a little knowledge is a dangerous thing.
In all three following scenarios assume the same exposure information for both users.
So, two people standing next to each other both with brand new 300mm f2.8 IS MkII's, one with a 7D mkII the other with a 5D MkIII. They sit in a hide together and across a field there is a fence, a bird lands on the fence, the crop camera owner has tighter framing but still needs a small crop, the ff owner needs to crop much more to get the same framing but so what, it is a nice shot. Who has less dof? Neither, dof in both images is identical.
Same scenario but both guys decide their wider shot is actually a nice environmental shot that shows the birds habitat well and the light is just catching the fence nicely, obviously the ff camera shows more fov view but both images work well, so, same place, same lens, same aperture, different sized sensors and therefore different framing, who has less dof? The crop camera. Why? Not because it is a crop camera (or because he is using a mythical 480mm lens), but because the captured data is enlarged more, the bigger you make something the less sharp it is, dof is about apparent sharpness so the more you enlarge something the less dof it has.
Same scenario, the bird flies to a second fence much closer to the hide. The ff user can still use his 300 for a frame filling shot of the bird, the crop camera user has to swap out his 300 to a 70-200 to get the same framing, he zooms to about 190mm to get the same framing as the ff user and they both take the same framed shot from the same place with the same camera settings, just different focal length. Who has less dof? The ff user, at the same distance a 300mm at any aperture will give you less dof that a 190 at that same aperture, this difference will be greater than the difference between the enlargement amounts.
So depending on the specific scenario the dof of a crop camera can be shown to be the same as, or narrower, or deeper, than a ff camera.
Further, "compression" is a very bad way of expressing perspective. Compression is dependent on where you are in relation to your subject, nothing else, the lens does not create "compression". In the three scenarios above who has the most compression in each situation? Neither, they are both shot from the same place in relation to the subject so all the images have the same perspective. That means the "compression" is the same in all those above situations.