We’ve discussed here a few times the qualitative differences in the photographs Pentax’s manual-focus 50mm lenses take. The general consensus is that all three lenses — f/2, f/1.7, and f/1.4 — are very good. But the f/1.7 is probably the best-liked of them for how it renders color and for its bokeh. Nobody would scoff at the f/2 if the f/1.7 wasn’t available; it imparts a wonderful warmth to color photographs and has all the sharpness anybody could ever want. While the f/1.4 is a fine lens, it tends toward clinical accuracy and so lacks the endearing character of the f/2 and f/1.7. It is also a lot more expensive than the f/1.7 for only a fractional increase in aperture.
While rummaging through my gear I found a little booklet from the 1970s SMC Pentax-M era that describes and shows the optical design of all the lenses Pentax sold then. It’s fascinating to see how these lenses are designed. All of them are some number of elements in two groups; all of them have similar front-group design. It’s the rear group that differs most among them. Check it out:
This book is about the SMC Pentax-M line of lenses, but I feel sure the corresponding SMC Pentax-A lenses are optically identical. The Pentax-As seem to be lighter, probably due to more plastic in the bodies, but they allow later bodies to control the aperture.
Here are three shots, one from each lens. Sure, film, lighting, processing, and scanning varied, all of which play into the final results. But really, can you tell which lens shot which? Probably not, and that’s the point: get any of these lenses and you should be quite happy.