Archived posting to the Leica Users Group, 1998/05/01
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]My message was interupted by an internet gremlin. As I was saying, a theoretically perfect lens is at its sharpest when wide open. Consult a good photographic optics text book for the details. In practice, even Leica lenses do not approach perfection. Aberations of many sorts tend to decrease when stopped down. At the same time, diffraction effects caused by the light passing the edge of the diaphragm increase. The conjoint effect of both factors results in maximum sharpness when the lens is stopped down two or three stops from maximum aperture. For an F2.0 lens, this would be about F4.0. The maximum sharpness region continues to about F8 at which point diffraction effects begin to predominate. About F16 is the smallest stop for most normal lenses that gives acceptable results. Experiment a little with different stops and you will see what I mean. - LarryZ