</span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (David A. Goldfarb @ Dec 13 2002, 03:23 PM)</td></tr><tr><td id='QUOTE'>
I think the explanation is not that the focal length of the lens actually changes, but that it is an illusion resulting from uncorrected spherical abberation being corrected when the lens is stopped down.</td></tr></table><span id='postcolor'>
I agree. Having worked in Optical Quality Control, I have been *very* sensitive to careful identifcation of optical properties and specific errors. *Apparent* quality can shift significantly as the parameters of testing change. A smaller aperture will significanly increase the
"depth" of focus- properly known as "hyperfocal distance", and edge-to-edge resolution, by restricting the area open to both spherical and chromatic abberations, curvature of field... (and a few other factors) until diffraction effects kick in.
Incidentally, one of the most valuable accessories I own are 40.5mm "Soft" filters. I bought two of them solely out of curiosity from a "bargain box" as they were the precise size to fit my Rodenstock enlarging lenses. Indispensable for printing Portraits... they do a remarkable job in eliminating minor (and some not-so-minor) skin flaws. I *do* have to remember to remove them when I focus.
I prefer to keep the sharpness in the negative (i.e. not using "Softars") and "adjust" as necessary in the enlarger.