Glenn argues that the 600mm gives 1.44x more magnification than the 500 (i.e. (600/500)^2) not 1.2 (600/500). But, is that relevant? The resolution of a lens, the most important factor, is a linear function of focal length, not of length squared. For example, a 600mm lens will give the same size image as a 500mm at 20% further away, not 44% further. When you use binoculars, you talk about x8 vs x10, i.e. the linear magnification, not x64 vs x100, the square. A 600mm lens would seem to me to be a 20% increase in reach, not 44%.