Some photographers say that this extra reach amounts to a crop of the full frame—equivalent to simply blowing up the center portion of the full-frame image.
But this is a film analogy that doesn’t hold true in the digital world. The APS-C camera concentrates all its pixels in that smaller frame, whereas if you crop the image from a full-frame camera, you lose pixels.
For example, if you put a 200mm lens on a 12.1MP full-frame Nikon D3, then cropped the picture to a field of view equivalent to what you’d get using a 300mm lens, your image would wind up just 5MP in size. But if you put the same lens on a 12.3MP APS-C sized Nikon D300, you’d get a 300mm-equivalent image—with the full 12.3 megapixels.
With today’s high-resolution full-framers, that compromise isn’t as striking, but it doesn’t disappear. Put a 200mm lens on a 24.6MP full-frame Sony Alpha 900, then crop, and you’d get a 10.5MP image—respectable, but still less than you’d get with a 12.2MP APS-C Sony Alpha 700.
So the telephoto advantage of smaller-format DSLRs is very real. Sports and wildlife shooters in particular can benefit from a smaller-sensor DSLR, either by getting long reach with relatively compact lenses, or by getting huge reach with big “legacy” glass—full-frame lenses originally developed for film SLRs