Originally posted by SOldBear Pentax-F series is auto-focus. Technically there is no difference between F and FA.
Some people prefer F series. They say that the F series is better built. I don't know about that, but my F50 1.7 does feel heavier than my FA50 1.7.
It's not exactly true that there is no difference but from a practical standpoint you get pretty much full functionality on D-SLRs using F (KAF) as well as FA. (KAF2) The FA (KAF2) mount added support for MTF program line and data and contacts related to power zoom.
It's hard to completely generalize but the F lenses tended to have a little more metal and a little less plastic. Some of the F designs were direct autofocus ports of previous Pentax-A manual-focus designs. The F's tend to be a little more rugged-feeling.
At that time, it doesn't seem that there were any real cheapy Pentax-F SMC lenses. This market was instead served by various non-SMC Pentax-F and "Takumar" releases--some of these designs were similar to Pentax-F SMC but may have had some cheaper materials & finish, lacked SMC coatings, etc.
The F designs, being a bit older tended to lack some things that we might take for granted in the latest designs. They generally didn't come with lens hoods at all and when hoods were available they usually were screw-in rather than bayonet designs. Furthermore, most had rotating front elements when focusing, which is less friendly for polarizers and also means that hoods were round rather than using more efficient petal designs. They generally didn't focus as closely and when close-focus was available, it was (like with Pentax-A) often at one end of the zoom and involved entering a 'macro mode' on the zoom ring. This can be problematic at times because you may only be able to focus closely at 70mm but minimum focus distance might be quite long at the short end of the zoom.
FA designs gradually adopted some of these more modern features--internal focus, bayonet hoods, closer-focusing throughout the focal length range. As zooms became the standard "kit" lenses sold with autofocus cameras however, more cheap kit zooms were released with a priority on low cost, size & weight, often with slower f/4-5.6 apertures, sometimes with plastic bayonet mounts, etc.