Archived posting to the Leica Users Group, 2009/12/07
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Richard Man wrote: > Ah!! Duh. OK, now it makes sense :-) I will check later on today. > > Basically, due to the somewhat imprecise and tolerance of various > factor, the focus is "optimized" for one aperture. In some lens > design, the focus remain constant even when the aperture changes, > whereas some the focus changes a bit, which usually is hidden by the > DoF or the film unflatness etc. Focus shift is caused by spherical aberation. Less spherical aberation = less focus shift. Spherical aberation is seen as different planes of focus for light rays from different regions of the lens, i.e., central or outer regions. With spherical aberation, at the widest apertures there's never a precise focus 'plane', it's more like a zone of reasonably good focus: a blending of the central and outer rays' focus planes. When the lens is stopped down the influence of the outer light rays is removed so the zone of good focus becomes more sharply defined and since it's influenced more by the central light rays, it shifts toward the the central light rays' plane of focus. Doug Herr Birdman of Sacramento http://www.wildlightphoto.com -------------------------------------------------------------------- mail2web.com - Microsoft? Exchange solutions from a leading provider - http://link.mail2web.com/Business/Exchange