I use the 50 mm and 80 mm Rodagons in my enlarger. These are said of being good lenses. I checked how much the need stopped down.
You may work stopped down one and a half stop, and I do so for larger prints and slow papers. But the highest sharpness in the corners I get only if I stop down two stops.
I checked diffraction effects not during printing, but during shooting. The optical laws are the same. Degradation starts at f/11 (50mm) or f/16 (80 mm), but this is just remarkable. One or two stops more closed it can be seen easily.
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