Middle grade lenses like Componars, perform best about 1-1/2 to 2 stops down. My 50 mm Componar came with a recommendation for use between f5.6 and f8. That said, when making small prints, lower magnification, it's less critical. Larger prints, much more critical. APO lenses are better corrected for flat field and often perform well wide open. I attended a lecture given by a Leitz optical technician. He said, all photographic lenses are sharpest wide open, but in the case of enlarging lenses, they may not be sharp out at the edges. Stopping down a stop or two will crisp up the edges at a minimal loss of overall sharpness. I certainly saw this phenomenon while photographing circuit boards for ITT. When I first began, I always used f45 on a 135 mm lens on a 4 x 5. This was true macro photography, where the image on the ground glass was same size or larger. I was a little disappointed in the results. ITT had several optical engineers working there. Don Taniguchi was the guy that made the photo resists for the tiny circuit boards. He said, try f16, and bingo, like night and day. Too small an aperture when doing close focusing, which is what most enlarging is, can create diffraction causing a loss of sharpness. I use f5.6 mostly and change the time.
