Thanks, I just picked up a 50mm M macro f4 at the weekend on ebay, and think I am going to get the M 100mm f2.8 and sell my M 135mm. I understand why the 85mm lenses command so much, as the M 85mm f2 is a full stop faster than the 100mm, and is going to be better for portraits indoors. I'm staying on brand as I really like the build of the old M series.
Interestingly, the Pentax literature says about the M 85: "Creates a soft focus effect in the areas just beyond the depth of field." Don't all lenses do that?! Seems like the 85mm was deliberately designed for portraits. In the Quality in Photography book by Hicks and Schultz, it says that lenses are optimised for a particular object distance. Is this really the case or a popular myth? I seem to remember reading the contrary on Leica's website recently.
EDIT; just picked up the M 50mm macro...now all I need is the M 85mm