Nikon is a greedy company. They make more money if you have to buy new lenses every time you buy a new camera. So, unlike Canon and Pentax and other camera manufacturers, incompatibilities are built into the cameras. Today's entry level AF-S mount cameras can't use yesterdays AF mount lenses. The Nikon D5100 is an entry level AF-S mount camera and it can't autofocus with an AF lens. It's especially sad because one of Nikon's best and more affordable lenses ever is the AF-D 50mm f/1.8 lens. But you can't use it on your D5100. You have to buy the more expensive AF-S 50mm lens, which has more distortion and is not as good. However, Nikon's pro cameras can focus and meter with a wider range of Nikon lenses. So it's particularly insulting to the entry level camera owners. I ditched Nikon for this very reason and stick with Pentax. Here is some sweet irony - the Nikon D5100 can't meter a Nikon AI manual focus lens, but a Canon EOS DSLR can use that lens (with an adapter) and it will happily meter with it too. In this case Canon is more compatible with Nikon lenses than Nikon itself is !!! Nikon's lens incompatibility game is all about greed. Incidentally, mirrorless cameras are very popular for fans of manual focus lenses. People like to adapt manual focus lenses to use with their mirrorless cameras. For example, I like to use Hexanon lenses on my Olympus EPL1 mirrorless camera. Nikon has a mirrorless camera, and it is the only one that will not meter with adapted lenses !!! Here again Nikon is showing us that it wants to force it's consumers to buy from a narrow selection of lenses. Nikon is greedy and you Nikon shooters have less choices in lens selection as a result. Canon, Pentax, Sony, Olympus, anyone is better. Nikon is the greedy fat bastard in the group.