Pentax currents offers a choice of three medium focal length lenses: the P-FA 50mm f/1.4, which lists for $450; the P-D FA 50mm f/2.8 macro, which lists for $650; and the DA STAR 55mm f/1.4 SDM, which lists for $1466. Even though the 55mm f/1.4 SDM sells for closer to $650 and the 50mm f/1.4 P-FA sells for closer to $360, these are still relatively high prices. Nikon and Canon both offer a 50mm f/1.8 lens for less than $125.00. For those of you tempted to drag out that old "you get what you pay for" argument I suggest you read further.
On a whim, I decided to see what sort of results I could get by mounting an older manual focus Pentax-M 50mm f/1.7 on my Pentax K-7. This lens was produced from 1977-1984 and sold as the equivalent of today's "kit lens." That is to say it was nothing special and practically every basic Pentax camera kit had one. Pentax produced tens of thousands of them. Mine was mounted on a Pentax ME a friend gave me a few years ago because no longer had any use for it. Frankly, it spends most of its time in my camera closet, unused--so in short, if you can't find one of these in excellent condition for less than $50, you simply aren't trying.
Back to the results. As you can see from these sample photos, the 50mm f/1.7 is certainly not lacking in resolving power, image contrast, or beautiful bokeh. I've never used the Pentax P-FA 50mm f/1.4 or 50mm f/2.8 macro, so I can't comment on how it compares to these two, but I do have the 55mm f/1.4 SDM, one of Pentax's very best lenses, and I can tell you from personal experience that the 50mm f/1.7 does not suffer in comparison. In fact, aperture for aperture, you'd be hard-pressed to see much difference between them. And did I mention that the 55mm f/1.4 costs at least $600 more?
Of course there's more to the story than optical quality alone. The Pentax-M 50mm f/1.7 is a manual-focus lens, so it doesn't have focusing speed in its favor. Ironically enough, the 55mm f/1.4 SDM is no speed demon either. As for accuracy, the 50mm f/1.7 was spot-on while the 55mm f/1.4 required microfocus adjustment to correct for back-focus. Once adjusted, however, the 55mm focussed equally well, even at f/1.4. Other factors in the 50mm f/1.7's favor are its small size (it accepts 49mm filters vs. the 58mm for the f/1.4) and light weight. Their build quality, though different, is equally confidence-inspiring.
Let's be clear: I'm not trying to discourage anyone from buying the Pentax 55mm f/1.4 SDM. It's an excellent lens and I have no argument with Pentax pricing it to make a profit. All I'm saying is that if you have a hankering for a top-quality 50mm lens but you can't or don't want to spend hundreds of dollars to get one, you'd be wise to snap up the first Pentax-M 50mm f/1.7 in good condition you see. It's that good.