One of the benefits of shooting with Pentax is the ability to use lenses from a larger format on smaller cameras. With an adapter, you can use Pentax 67 lenses on the Pentax 645 or Pentax 35mm cameras. You can read more about this in previous blog posts here and here.
The crucial question is would the old school lenses stand up to the high resolution images created by the 645D. I was really pleased with the results I was getting when using the 67 105mm lens, but unsure about the telephoto 165mm. I’d tried it once or twice but found the images soft. As I recently acquired a 150mm 645 lens I could compare lenses of similar focal lengths and see what the differences were.
I taped a page from a catalogue to a concrete post about 3 meters away, then took a shot of the page using both the 645 150mm lens and the 67 165mm lens with 67 to 645 adapter. I also photographed the page at three different f stops: f11, f2.8 and f22. I’d then compare the images at 100%.
Both lenses performed best at f11. Both lenses also performed very well at f22. At 2.8 the level of sharpness in the 645 150mm was reduced to a small degree and significantly when using the 67 165mm
At optimal apertures the 165 lens with adapter worked great. If you primarily use mid to small apertures when shooting landscapes or in the studio then your old 67 lenses will continue to give you faithful service on the 645D. If you usually shoot wide open then in this case you’d be better off with the 645 lens.