In his book Post Exposure Ctein suggests that "adequately sharp" (meaning, it appear sharp when viewed alone) requires about 5 lp/mm on an 8x10 print, but that "perfectly sharp" (meaning no difference is visible in side-by-side comparisons if sharpness is increased further) requires about 30 lp/mm! On an uncropped 35mm negative (enlargement factor 7) this corresponds to CoC diameters of about 0.014 mm and 0.002 mm respectively, but the latter is not achievable due to film, lens sharpness and DOF limitations. For "adequately sharp", an aperture of f/16 should give an airy (diffraction) disk diameter of around 0.014mm.
My own inclination is to retain diffraction sharpness and compromise DOF if necessary since I can adjust the focus distance to ensure that the aspects of the scene that I consider most important - typically those closest to the camera - are as sharp as possible, while allowing some loss of sharpness for (due to DOF limitations) in other areas. If you start to lose sharpness due to diffraction, then this affects everything in the photograph, which I find the worse of the two evils.
Last edited by andrew.roos; 04-04-2012 at 03:16 PM. Click to view previous post history.