If you can find a Pentax F or FA 50mm macro 2.8, by it!! A very good lens for close up work.
Mike
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K10D AF lenses: Sigma 18-50EX macro:FA20 F2.8:FA35 F2.0:FA50 1.4 & 2.8 macro: FA28-70 AL 4.0: F35-135 + K,M and A lenses from 15 to 400mm http://www.wavecurrent.pixbox.se/
I would suggest the Tamron Di 90mm Macro..Brutally Sharp!!!
After that the new Pentax DA 100 f2.8 Macro. I had the Tamron when using Canon and when switching to Pentax got the Pentax Macro. Both excellent, but my Nod for the Tammy.
Another vote for the Tamron 90mm/2.8 Di. I just got mine last week and it isn't just sharp -- it's "oh....my....gawd" sharp. And until the end of the month, Tamron has a $90 mail-in rebate on it.
I have both the Pentax M 50 f4 and A 100 2.8 macros. The M 50 is an inexpensive (comparatively) great little macro and very, very sharp. With extension tubes or bellows it will get you very close. If you don’t need the speed this lens is a bargain. The A 100 macro is pretty rare and very sharp as well. It is a true macro in that it will give you 1:1 reproduction without extension tubes. The extra length and speed can be very important in macro. Length and speed come at fairly high cost in the world of macros. From my personal experience I would recommend either of these lenses.
Tom G
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Pentax K2, MX, K10D, D-BG2 grip, Katz Eye screen. K 24 f2.8, K135 f 3.5, K200 f2.5, K300 f4., M 28 f 2.8, M35 f2.0, M50 f4.0 macro, M50 f 1.14, M150 f3.5, M200 f4.0, M75-150 f 4.0 zoom, M80-200 f4.5 zoom, A 100 2.8 macro, DA 70mm f 2.4 limited, Auto bellows M,, Refconverter M, Refconverter II
Pentax A* 200mm f4 Macro definitely belongs to the dream list!
But to the question, consider the focal length against your kind of usage, before buying! I have both 70mm and 125mm and like them both. They have their uses.
The best macro is one you can 1) obtain 2) afford 3) use a lot. Any of the above mentioned lenses (and probably a few more, plus a bellows setup) is more than capable of totally awesome shots.
I should know: I have to learn to shoot macro well, and one lens over another won't help me there