</span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Ed Sukach @ Dec 13 2002, 11:01 PM)</td></tr><tr><td id='QUOTE'>I have heard this idea of focus "shifting" at different apertures many times, but from my training in physical optics it is difficult to explain.</td></tr></table><span id='postcolor'>
Ed,

this is not a real focus shift, it is an apparent focus shift. Some aberrations can actually be diminished by a slight defocus (i.e. an offset to the extension computed by the exact focal length and the exact magnification ratio/subject distance). Some of these aberrations are influenced by the aperture diameter. So stopping down a lens can shift the apparent focus towards real focus (usually not the other way round). David probably meant the same thing. Depending on design goals, lenses show different behaviors here. The primary design goals of enlarger lens are: resolution and flat field. Camera lenses usually have other or additional design goals, resulting in different focus-shift behavior, too