Lots of good advice here, so some of what follows is redundant.
1) for indoor, studio work, a shorter FL is more useful. Many excellent macros in the ~50mm length, but probably the 50mm SMCA f2.8 is as good or better than any.
2) as someone noted, FL's of 90-105mm are the most versatile and the minimum required for live, in-the-field insect macro. Almost any lens, any manufacturer in this FL range will be excellent. I have an ancient Tokina (requires a matched adapter to reach 1:1, purchased in the 1980's as I recall) that is excellent. All versions of Tamron's 90mm macro have an excellent reputation, ditto for Sigma's 105mm macro.
3) If you plan to do a lot of wild insect macros, you will eventually desire a 180~200mm macro. Sadly, there are no macros at this focal length currently in production for K-mount. I have an original 200mm f4 SMCA-ED purchased used about 1991. It is outstanding, but very hard to obtain and only at wallet-deflating prices. Rumor has it that Pentax will reissue the AF version of their 200mm macro around the time they introduce a FF body, but do not hold your breath.
4) AND, AND, if you have a good quality SFL lens of 50~200mm, or a good quality zoom that has a filter thread no larger than 67mm, consider purchasing a RAYNOX CM-2000 MACRO EXPLORER SET ($103). This pair of achromatic clip-on filters can provide excellent quality macro images (track down the thread here on PF that is devoted to these filters). This is by far the least expensive way to get started in macro, and if you have a good SFL lens, the results can be outstanding. I have used these on an old 200mm SMCA lens (purchased for $100) and found the IQ embarrassingly competitive with any of the macro lenses I own. BTW: Results with a zoom lens are unpredictable - excellent with some, unacceptable with others; using these on a longer FL lens increases magnification, but
does not increase the lens-to-subject distance; and to focus, you MUST move the entire camera + lens fore-aft (turning the focus ring is ineffective).
RE: You DO NOT need AF for macro, and at high magnifications and/or shorter focal lengths, AF may not work. At high magnifications (around 1:3) and using a shorter FL (especially ~50mm) you will probably want or need some kind of macro-focus rail. I find that flash illuminated macros are sharper than natural light macros about 80% of the time, even when the camera is atop a heavy tripod. With flash, broadly diffused lighting is best, and flat, straight-on lighting is commonly best. Modeling with shadows, a concept that is a legacy of B&W studio portraiture, doesn't always apply to macros. Extreme color macros of insects emphasize details of hairs, and shadows of those hairs can cause visual confusion that actually detracts from image impact.
There is good advice on macrophotography here on PF, and there are several excellent sites devoted to macro. Here are two that I like:
www.extreme-macro.co.uk/ www.photomacrography.net
The first site is especially devoted to macro at greater than life size using image stacking on dead subjects, but there is all manner of useful information on lenses, flash, technique, and the site's author uses Pentax.
The second site is very extensive with many images posted by readers, basic to very advanced advice, an equipment exchange, macros from 1:5 or so up to 10X or more, etc. etc. The cannot-be-done-better quality of some posted images can be either inspiring or intimidating.
Traditional advice: start simple; take pictures to please yourself.
Originally posted by Venturi 
Post #20 by WPRESTO
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