Good Morning, Barry,

There has long been "rule" that lenses tend to perform best a couple of clicks down from maximum aperture. That may well be correct in some (many? most?) cases, but I wonder if isn't also a holdover from decades ago when some lenses were less precisely constructed and/or there was more variablity from sample to sample. Clearly, a lot of lenses on old folding cameras have lenses which are rather poor wide open but respectable when stopped down somewhat. The problem with stopping down too much is that an extremely small aperture can cause diffraction which degrades an image. This is one reason that a lot of 50mm lenses on 35mm cameras have a minimum aperture of only ƒ16. On many large-format lenses, apertures down to ƒ45 or ƒ64 are not uncommon, and diffraction isn't much of a problem since the diameter of the opening is still large enough to minimize diffraction. Printing at ƒ8 with a 50mm enlarging lens makes some sense, although I haven't noticed any problem when I occasionally use ƒ11 or ƒ16. With the 135mm or 150mm enlarging lenses used for 4 x 5, an aperture of ƒ16 or even smaller still has an opening which is large enough to avoid any significant diffraction and allows a long enough exposure for dodging and burning.