I have both the 300mm F2.8 Nikkor Ais and the 300mm F4 Nikkor AF (the first generation of autofocus Nikon. Nothing really needs to be said about the 2.8; its reputation is well established. Its only downside, if it can be considered as such, is its huge size; out for a day's shooting, it can be a bit of a burden to lug around (backpack recommended). The 300mm F4 AF that I use is stellar optically (good contrast, very, very sharp). Its only shortcoming, aside from the slowness of the autofocus, is its minimum focusing distance - a little over two meters. So how do I choose which to use? For wildlife or sports, or for events where I'm unlikely to have to do a lot of walking around, or stray to far from my car, the nod will go to the 2.8. For out wandering around for the day in the city, the F4 will do the job. On balance, both strong performers; circumstance determines which one comes out.