The 28/2.8 AI-S is so-so as for its innards mechanics (polycarbonate barrel, CRC unit made of cheap white metal) and as a result most of them suffer from wobble and play to a point which may cause blurry images because the body aperture preselection fork will make the wobbly lens optical unit vibrate when the shutter is triggered (too bad given what the lens offers optically wise). If you get one with some internal wobble and play (very fairly common), have it serviced prior using it so that it comes back tight all around with a firm focusing action and no rattling noise when you gently shake it.