Interchangeable backs is the way to go but I started with the Pentax K1000 and was so impressed that when I moved to MF I stayed with Pentax and got the 67II. Then one day I decided to "replace" the 35mm system with the 645N for the negative size and portability. At the time the 645N the logical choice since I had amassed a collection of 67 lens and only needed the 67 to 645 adapter which I got. Too bad Pentax didn't engineer the 645 to take interchangeable backs but I have been very satisfied with their product line and their MF lenses are outstanding.
Thomas
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