JCO, If you have a M42 camera then it may make sense to consider that the Takumar is an advantage. Do you have any information that suggests the K and Takumar versions have different optics and/or apertures ? My comments are based on the following quotation from Alex Nemerovsky's site:
"K500/4.5 feature very good optical design as well as a preset aperture. It's quite heavy and, unless you want to strain your back and get a fuzzy shot, it should be tripod mounted most of the time. This lens has the same design as the previous Takumar version of 500mm. K500/4.5 lens has a 52mm rear filter." (http://home.att.net/~alnem/html/pentax_primes.html#500) Jim www.jcolwell.ca P.S. the SMC 500/4.5 has a manual aperture, not preset - maybe the other comments are also incorrect(?).
