[johnha]

I was very impressed with the results from P67 500mm f/5.6 on my 645D when I did some test shots: Test shots from a 645D & SMC 67 500mm f/5.6 with 100% crops - PentaxForums.com

Ok they're not full frame 645 and the mirror will be smaller than a 645n, but in terms of IQ I can't fault the coverage on 33x44.

The key to the results seems due to the clear day and good atmospherics (which I didn't really appreciate at the time). Since shooting them last week, on no days since has it been as clear. It's not something I noticed in the finder when l took the shots.

I haven't tried it on my P67 yet, it's quite a heavy lump to lug about and setup, but once on the tripod it handles very well for its bulk.