Rob wrote: DOF is only perception and in addition to the common guide formulas I have found that unsharp lenses seem to display a more broad DOF than a sharp lens where the absolute plane of focus is apparent.
REPLY: True as sharpness is relative (within reason). However, this also explains partly why lens quality isn't such an issue for medium format. Due to the fact that DOF is less for the same angle of view as 35mm, for the same image more of the subject is farther away from the plane of focus due to smaller aperture and hence less sharp than otherwise (only the plane of focus is truly sharp). Also, you may risk to have to stop the lens so much down that sharpness reduction due to refraction comes into play. Lens quality also have less overall input to the resultant resolution (lens + film resolution) the larger the format. The larger the format the less magnification of the lens resolution/imperfections is needed for a certain end result. Also, larger formats are genreally less flat than smaller film formats contributing to loss of sharpness. Hence, theres not much need to worry too much over optical quality for MF lenses. The DOF issue is the real problem for MF. Imagine wanting to take that landscape shot you usually do with your 24mm lens for the 35mm format; everything from your shoetips to the mountains in the distance in focus. But the lens you are using (with the 24mm's angle of view) has the DOF of your 35mm system 50mm lens. Oooops! Pål
