Originally posted by aamendoza93 There is the safe option of buying the DA 50mm f/1.8 on amazon for $88 but I found the Pentax M 50mm f1.7 on ebay today. I looked at both reviews and I really don't know where to make my decision.
I have M 50mm f1.7 and its a great lens. But I got it back way before the DA 50mm f1.8 came out. Today, the DA 50mm f1.8 is sooo affordable, I would recommend it over the M 50mm. Basically the same optics, but DA has better lens coatings, rounded aperture blades (M 50mm makes a star and hexagon swirl between f2 and f3.2; DA 50 keeps it round all the way), auto-aperture, and autofocus.
The M 50mm has advantage that it has aperture ring and distance scales - this means it can be easily used on older manual focus SLR cameras, on extension tubes, and so on. But on a modern DSLR, I would recommend you buy A 50mm f1.7 or newer - basically a lens with A on aperture ring, as that makes it much easier to use! So I vote for DA 50mm f1.8. OR DA 40mm XS, OR DA 35mm f2.4 OR FA 50mm f1.4. These are the most affordable, yet still amazingly high quality primes to start with. If you buy manual glass, I suggest you get the kind that is absent in the DA lineup, like 24mm, 28mm, 85mm, 135mm..
Now, bokeh.. Biz up above wrote some numbers and stuff, but I think those could be easily misunderstood. Basically, to get apparent "big bokeh" you need a) minimum focus distance, b) lowest f-number, c) telephoto lens (higher # of mm, longest zoom on zoom lenses). 50mm is fine on APSC. Even 35mm can do it well, if you are close enough to the subject. DA 50mm f1.8 will give you more apparent bokeh at f1.8 than DA* 50-135mm would at 50m f2.8. But if you put DA 50mm to f2.8 then it would be virtually the same (sure, a little different shape, smoothness, contrast, and so on; different characteristics, but the same DoF and "size" of blur). Yes, there is a significant difference between 2.8 and f1.8. Moreso with f1.4. DA 40mm XS - a lens that is not really known for its bokeh, it is only f2.8, but for some reason, I like its bokeh. Rounded aperture blades, super compact lens (think it was smallest DSLR lens ever made when it came out, probably still is) and sharp!
tl;dr: Buy M 50mm f1.7 if you love mechanical lenses and don't mind doing extra work for the shot (also, don't pay over $60 for it) - keep in mind this lens is what, 30 years old? Buy DA 50mm f1.8 because it is affordable, fully automatic, comes with warranty, and has modern coatings. Buy A 50mm f1.4 as compromise to get some automation and some more bokeh smoothness than with f1.8 and f2.8 lenses. FA 50mm f1.4 is another great option, but costs more. It is best of both worlds, as it gives full automation and fast aperture. But where I live, it is absurdly expensive and rare.. I recommend DA 50mm, all things considered.