If I am looking for a new lens for my Nikon D3100, What am I looking for to tell me it will fit my camera. Vague? OK. If I am out somewhere ( not very often lately without my camera ). But lets say I am. I find at a Garage sale, Boot sale etc. a Nikon lens that is, at a to die for price. How do I know, that, that lens will fit my camera. Obviously a size or number somewhere, but where.