Hi, I am about to start using my recently acquired set of Pentax A series 645s on a Canon 5D (my Zörk shift adapter is on its way). Does anyone have a feel for how far I could stop these lenses down before diffraction becomes a noticeable issue with this set up? I will be mainly using a 35mm and 45mm, but I also have a 75mm and 150mm. I want to be able to print up to A2 (16x24 inches).
Peter,
Most lenses are at there best 1-2 stops down from open and 3 stops as far as wide angle lenses are concerned. This topping down eliminates the refraction issue. Past that, diffraction will rear its head vis-a-vis the amount of enlargement.
Now the cloudy issue--Pentax did not do such a good job with the 645 A lenses, perhaps to keep prices down, compared to their great 6x7 Pentax lenses. So I would try to stay at the F stop parameters I stated above, as after about 11-16 diffraction really sets in, and significant softening of the image will occur.
Your 5D will require 16x magnification to reach 16x20. with the maximum resoluton of the human eye usually stated at 8 lines per MM, using the 8 lines times the magification, 16x., you get 128 Lpm as your goal for resolution required for a tack sharp 16x20.
You need to be near 4-5.6 aperture for that size...
Testing will tell.
Rod
