I know this subject has been tossed around here (even I have discussed it), but I am having a difficult time recalling some very informative opinions that were expressed here. Anyway, I do not mind using pre-AI lenses (I love their bulid quality), and AI lenses are fine, too. I tend to like a 50 that shows little or no distortion and that displays a good color range. I have used the Nippon Kogaku S 50/2, the pre-AI K 50/1.4, and the 50/1.8 AIS. Ultimately, speed is not of the essence, and so an F/2 lens is usually fine for most of my applications.