The high qulaity enlarger lenses offered by Schneider, Rodenstock, Nikon and Leica are at their best optically at f5.6 - f8. The f2.8 lenses offer top resolution across the field at f5.6. At f8 diffusion begans to take affect, and at f11 and beyond it definitely reduces resolution and contrast to the extent that is visable in large prints. Depth of field with these lenses is not important since they are flat field lenses, and if the negative and the printing paper are flat, they will all be in the plain of focus. Use a focusing aid. Vignetting should not be visable with these lenses at f5.6 and at smaller apertures.(posted 9560 days ago)The effect of stopping down is easily demonstrated by observing fine detail in the projected negative image with a high powered grain magnifier.