[frank]

The Pentax 67's problems are also due to the shutter curtain. See the illustration, even with the mirror locked up, here:
http://www.luminous-landscape.com/re...ntax67ii.shtml

Thanks for that link. Here's a quote from that site:

The problem I experienced requires some further explanation. The Pentax has a large rapid-return mirror. At slow shutter speeds (below 1/125 second) and especially with long lenses, it should be locked up. This is SOP (Standard Operating Procedure) for all medium format cameras, and should be even for 35mm cameras. Mirror shock is a real sharpness thief for critical work.