I have both Schnieder and Rodenstock lenses on my Besler enlarger. My question is in regards to the use of the f-stop on the lenses. Is the purpose to control the time exposure or focus by adjusting the aperature. Will an enlarging lense give me a sharper image at f11 than at f4?

-- Joel (figtrees@earthlink.net), December 31, 1998

Answers

Most lenses give their best performance at two or three stops down from wide open. Enlarging lenses are no exception. A curved negative (not a problem with a glass carrier) will show more sharpness in the corners with a small aperture, because of increased depth of field, but you begin to loose sharpness with diffraction at minimum apertures. If you focus with a grain magnifier, you can sometimes see the effects as you stop down. Check out "Postexposure" by Ctien, an exhaustive examination of enlarging issues.

-- Phil Stiles (pjs@worldpath.net), January 01, 1999.

Exposure should be controlled by time, not aperture. The maximum aperture should be used for setting up, including focusing, but not for exposure.

35mm camera lenses are designed to give 'reasonable' performance at largest apertures. Enlarging lenses (and Large Format lenses) do not have this contraint. The manufacturer dosn't care about performance at maximum aperture, and can concentrate on getting the best performance at smaller apertures.

-- Alan Gibson (Alan.Gibson@technologist.com), January 02, 1999.