I accidentally won both on Ebay auctions. The '77-84 M was $25. The '84-89 A was $60. Both are very sharp, a challenge to manually focus, and optically, from what I've seen, equivalent. They get very sharp around f3.3-4. There was some concern about whether there is a notch between 1.7 and 2.8. Indeed there is. Anyone know if that notch is f2? These are similar in FL to the Oly 45mm 1.8 and just as fast (45 is very sharp from f2.5, however). I kind of bought them to play with, see how I like legacy lenses, and to see if I want to buy the lighter, easier, faster 45 (I do). I have fun playing with the manual focus. I would definitely not want to take this lens anywhere where I'm shooting people or animals, however! Pentax A 50mm 1.7 - Usually goes for $80-100. This has auto-aperture if attached to the right camera. It's 20 grams lighter than the M. More plastic, I'm assuming. After reading all about nifty fifties, THIS is the lens I wanted. Pentax M 50mm 1.7 - Had I known I was going to win the A I wouldn't have I wouldn't have bid on this. I didn't want it. After using it, however, I prefer this to the A. Why? I can't say, really. The aperture ring is a bit smoother? Older is cooler? I'm actually going to sell the A and considering its condition and that it has caps, probably get $100 for it. I might start a thread on here first, however, to give it to someone who appreciates it!