I would say for the K10D and your style of photography (had a look in your gallery) I would give the Asahi/Pentax 1.8/55 a try (any model).
Now that you have that M42 adapter there's just a ton of nice lenses to put on.
If you are on a budget but have some patience, the easiest is the following: Look out for old 35mm film cameras in pawn shops, flea markets or the electronic flea markets. If you can get old east german cameras like Praktica (except the newer B-models with bayonet), Exa 1b or c, or the very old Contax SLR, you have an excellent chance that there is a 2.8/50 Tessar on it. Should cost you less than the lens alone.
The 2.8/50 Tessar is just a Tessar. That is 4 elements (greek: tessa=4) with the 2 back elements glued together (displayed in the old Zeiss logo).
Almost any 2.8/50 lens will have this design. Of these in M42, I really like the Industar 61LZ. Very funny lens is its slower Tessar brother, the
Industar 3.5/50.
You will find both of the Industars on old Zenith cameras (M42 SLRs). And finally, you will find on most Zenith cameras the M42 Helios 44 M ... and that is the lens you should really get imo (that's a multicoated Zeiss Biotar). It's a 58/2.0 double Gauss design and one of THE portrait lenses on film and digital.
Different models and factory logos: Mother Biotar and her soviet daughters (all @f/4)
Cheers, Georg
Last edited by georgweb; 08-18-2010 at 12:31 AM.