Nikon has always tweaked the 50mm formula. There are optical differences between an S (S.C) and an Ai and an Ai-s. How much and whether it matters to you is dependent on what you need. I find as another posted, that the S is really quite good for B&W, I use an S.C for most of my color (digital) work and my Ai-s seems to be better at 1.4, and also closer up. YMMV...