Ok i want a 50mm lens, ill just manual focus the little guy, its cheap enough i want to try. NOW, which one do i want, i look on ebay, and there are 50mm 1.8 AIS, and 1.8 50mm MF lenses, since im on a d40, does it make a diffrence if its MF? can i just save money since it wont auto focus? whats the diffrence between an AI vs. AIS, both mf? will a 35mm AI fit my d40. Im confused!