Originally posted by UncleVanya There are multiple 100mm macros from Pentax. The non-wr D FA is not a Tamron design. It leveraged the same optical design (generally) that started with the F 100 macro. The main difference is the presence of an aperture ring and a focus clamp.
There is an older FA 100/3.5 that is a Cosina design by all accounts.
Optically I doubt many could tell an image made from the 2.8 versions of the F / FA / & D FA vs the current D FA WR unless given a lot of time and practice. I owned the F for many many years.
I know that the optical design by the D-FA100/2,8 is the same optical design as the F100/2,8 Macro but I have a vague memory somewhere it was written that it was built by Tamron (as another few Pentax lenses)
I had the Cosina 100/3,5 Macro itself, a 1:2 Macro which in the German speaking region is called Yoghurt-Pot due to the plastic construction.
It isn't important if one can or can't tell the difference between the lenses you mentioned.
I mentioned the D-FA100WR because it is built like a limited, is WR and now is offered often cheaper than even the F-or FA Version.
The F- or FA Versions are much heavier, the FA Version has a very thin ring for adjusting sharpness, which is a nuisance because macro you often want to adjust manually and not via AF. I sold my F50/2,8 macro for the D-FA100MacroWR and still have the FA50/2,8 macro but without the limiter button which just fell off one day in the woods without myself noticing, when I came home it wasn't there anymore, so I would make a loss selling it and thus kept it. Both, the F and FA-Macros are heavy as well.
I wouldn't consider the Cosina or Pentax plastic-fantastic 3,5/100 macros, the AF on those is terrible and for a cheaper but better 1:2 macro-lens I would prefer the
SMC A-Pentax 50/2,8 Macro, which is better built, light, short and one of the best 50mm primes aside of its macro function.