Kind of a strange question, but I'd be interested in hearing theories on this. I'm shooting with a Contax 645 + Phase One P30. On waist-up and headshots, I can always nail sharp focus. With full length shots, however, the images almost always look a bit soft. It doesn't seem to be really "misfocused," because there's no area of critical sharpness on the full length shots I'm concerned with; rather, the whole image looks slightly soft. So, I was thinking camera shake, but there's no reason why I'd get more camera shake with the same camera and lens (and f-stop and shutter speed) on a full length shot than with a waist-up or headshot, is there? Same owuld go for subject movement; you'd expect to always see it if this was my problem.
So now I'm thinking that perhaps there's a problem with my lens in the far, but not near, distances. E.g., focusing is perfect from 5-7 feet away for a headshot or waist up, but focus is off at the 10-15 feet needed for a full lenght shot? (Do lenses behave this way? Can they be out of alignment only at certain distances?) Or perhaps it's the AF that's inaccurate int he far, but not near, distances? But again, what I'm seeing in these full length shots is not misfocuing (e.g., where the focus point is behind or in front of what I'm aiming for), but an overall softness or lack of sharp focus anywhere.
I almost always shoot with the 140mm lens, so I'm not 100% sure if my other lenses display this issue on full length shots. I guess I should do some "brick wall" tests at various focusing distances with various lenses...