Originally posted by TomB_tx I'll second the Pentax 50 or 100 macro lenses, as both are excellent. The 50 is also fine for all around use, and the 100 is also great for portraits. For macro work you need to stop down quite a bit to have any depth of focus, which means lower shutter speeds - so a small or tabletop tripod is handy. For a K1000 the Pentax-A lenses have no advantage over the Pentax-M lenses, which are less expensive. The -M 50 is an f4 lens, but a very good performer. Again, for macro f2.8 has so little depth of focus that you would stop down more than f4 anyway.
Agreed, but a few reasons why I'd still go with the Pentax-A f/2.8 over a Pentax-M f/4:
a) More forward compatible.
b) Brighter in viewfinder to aid with manual focusing.
c) Better lens performance one stop sooner; the f/2.8 reaches optimum sharpness starting around f/5.6 vs. the f/4 at f/8. True, with macro depth-of-field you'll want to be more in the f/8-f/11 range, but we're talking differences of very, very shallow vs. very shallow.
The best argument for the Pentax-M is price.