I use an IQ160 on my 501CM and Flexbody. High resolution MF digital backs are less than forgiving, so on these old V system bodies there's not much room for error, both in terms of the camera body and mirror being properly aligned, and the photographer's technique. Lenses also need to be chosen carefully. All of the old Zeiss glass is excellent but not all can meet the demands of sensors with 6 micron (and smaller) pixels. In my experience, the 40 IF, 100, 120 (for macro work), 180 and 250 SA render beautifully on my 60 Mp back. When I get everything right - focus, light, solid on the tripod etc. - the results are great. But it doesn't always come so easily.
John