Is the Pentax smc D FA Macro 100mm f/2.8 Weather-Resistant Macro lens compatible with the K-r body, and is this going to be a good buy for sharp close up insect/nature shots etc?
Yes, it is compatible. The Pentax K-r has the usual PENTAX K (KAF2) bayonet mount and the smc DA 100/2.8 WR has Pentax KAF. Nearly all Pentax K items are compatible to each other, including backwards compatibility for it's additional features.
is this going to be a good buy for sharp close up insect/nature shots etc?
It has a 1:1 magnification, which means that the object you are photographing has the same size on the sensor. The minimal focus length is 30.3cm, at 100 mm. Note that the camera has a crop factor of ~1.5x, so the focal length compared to 35mm film is 150mm. Also, as it is weather resistant, you can shoot in rain, humid and dusty conditions and in wet grass.
All Pentax K items are mostly compatible with each other. The two big exceptions are that 1) Pentax digital cameras don't have the lever needed to read the aperture-ring setting of M-series lenses, limiting auto-exposure functionality, and 2) Pentax D-series lenses don't have the larger image circle needed for full-frame camera bodies.– MarkOct 8, 2015 at 0:26
@Mark - The K-r isn't a full-frame camera... Worth knowing if he plans an upgrade to the forthcoming, and sadly too late for me, Pentax FF offering, but it's not a factor in compatibility with the K-r.– Joanne COct 8, 2015 at 2:40
DFA should be full-frame compatible, though, right?– mattdmOct 8, 2015 at 9:32
DFA lenses have full frame coverage, although the WR version lacks an apperture ring so there aren't many options for using it on a film camera, if Pentax ever release a full frame DSLR it should be fully compatible. It's DA lenses which are officially crop frame only, I say officially because some of them actually have full frame coverage through all or some of their range. Oct 19, 2015 at 9:59