Originally posted by lukelbd I had thought the F versions were the same optically as the FA; this is incorrect?
On paper they're optically identical, but in practice they never are. I think it's because the glass elements were manufactured slightly differently in each generation. In my experience, each generation has a distinct character of it's own - in other words, K glass looks different from M glass, which looks different from A glass, which looks different from FA glass, ...
I find the character of the F and A glass are the most similar to each other - F glass looks almost like A glass with AF added. And occasionally a particular F lens model may look like FA glass (or visa versa). Perhaps this is mostly with later model F lenses or early FA lenses, where they "crossover" somewhat. But in general each generation yields a distinctive appearance. If you're a discerning viewer, you can often identify the generation of the glass just by looking at the images it produces (if you study the
http://www.pentaxphotogallery.com/photos/gallery/query?camera=&lens=820 long enough you'll start to notice the differences, and then as you build your own lens collection you'll see they follow the same trends). On the surface you might describe the "look" as differences in attributes like color, contrast, or CAs, While these differences are subtle, they're identifiable, and in my opinion subtle differences can make the difference between an image people stop to admire and one they pass right over.
I never tried the FA50/1.4 (there was no point for me, since the F version was rated better, and I usually favor F glass anyway), but I owned the other 3. I'd already viewed enough images to know I didn't want the FA50/1.4. However, I thought my F50/1.4 was rather nice - that is, until I compared it to the FA43. To put them in perspective, the scale (from 1 to 10) from the perspective of a discerning, near-perfectionist would go something like this:
10 - none (this is reserved for lenses like the Sony Zeiss 135/1.8)
9 - DA*55 (nearly 10 for sharpness, but backgrounds are a mixed bag)
9 - K50/1.2 (OK sharpness, but excellent backgrounds)
8 - FA43 (perhaps the best color of any Pentax lens)
5.5 - F50/1.4 (nice, rich color)
5 - FA50/1.7 (overall pleasing images)
4.5 - F50/1.7
4 - FA50/1.4 (rating based on comparing many images online and comparing to lenses I owned)
2.5 to 3.5 - Excellent kit zooms like the F35-70/3.5-4.5, F70-210/4-5.6, or F24-50/4
2 - DA18-55 kit zoom
1 - Canon ~18-55 kit zooms (sorry Canon, your L glass is great, but not your kit lenses!)
So a "3" is a nice lens I still enjoy using, even though there's better glass out there. As you can see, while the AF 50's are good, they're not outstanding. So while you may still want to take the time to find the particular model you like better, keep in mind that the differences become mostly irrelevant when you compare them to the more expensive models.
[It should be noted that most of my shots, with any lens, are taken within 1 stop (or sometimes 2) of wide open, so my assessments should be viewed in that light]