Is the loss of sharpness associated with diffraction an absolute function of the ratio of focal length as related to aperture much the same as is depth of field, or are there other elements associated with this phenomena? if one anticipates shooting at smaller apertures and has a choice of lenses, one faster than the other but otherwise similar, will the slower lens be better at an aperture that might be the minimum setting on the fast lens but in in its mid range? I have always preferred faster lenses for their relative ease of focusing given their brighter image and shallow depth of field in the viewfinder, but am wondering if I should rethink this before making further purchases.

Regards,

DAB.