Originally posted by Real Ale 85mm - This size appears to have particularly good reviews for this type of work. There is one that has caught my eye in and was reviewed within this forum in December 2015. The review of the tester and owners were very good. Unfortunately whilst it identifies average cost of the lens it does not identify the make or where one can buy it. I note that Pentax do not have a fixed 85mm so I would be looking at 3rd party It is the SMC Pentax-M 85mm F2 Production years 1977 - 1984
Http://www.pentaxforums.com/lensreviews/SMC-Pentax-M-85mm-F2-Lens.html?revie...te=&reviewasc=0 I note that there is a comment that this lens was succeeded by a faster versions in the A and FA series. The only 85mm lens I have so far seen for Pentax that are not expensive are from Samyang and one sold by walimex pro.
The M85/2 is a nice lens with good bokeh. Took me some time to get used to it but a very nice lens.
Seb