Originally posted by creampuff Hate to break your thought bubble but the reality is your legacy MF lenses aren't up to scratch with the latest FF sensors. Just how many Nikon D800 users are still using legacy lenses? Most of the old lenses just cannot make it in terms of sharpness and resolution with the current sensors. Not so telling on APS-C but the shortcomings definitely show up on FF.
Depends on the lenses. My favorite lenses on my M9 are the Leica Summicrons I bought back in 1969-1970. People are astounded when I pull up 100% crops off the FF sensor. Ken Rockwell has shots taken with a 1930s Leica lens on his site that look incredible at 100%.
Since APS-C shots need to be enlarged more for the same size print than from a FF sensor, and since MF lenses are harder to focus on a APS-C dSLR finder than on a good FF finder, I'd expect FF prints to look better than the same lens on APS-C.
Many of these old lenses are plenty sharp enough, but some will cause more "purple fringing" due to the characteristics of digital sensors. However, my old Leica lenses have less of this problem than some of the modern Voigtlander-Cosina and Zeiss lenses I've tried, and less than I see on some new Pentax lenses.
After looking at 100% crops from my K-5 with both K and M42 lenses, I'd be very satisfied with them. The K-5 finder just is harder to focus well with MF, and the focus indicator only tells you that you are "close.".