On Jun 15, 2010, at 15:07, Chris Mitchell wrote: > Bob Sullivan wrote: >> I still don't understand how is is there at f22 but gone at f2.8, > > I've always rationalised it as small aperture = large depth of field. Spots > come nearly into focus at f11 and smaller. At 2.8 they're so out of focus > they don't register. Is this right anyone? >
At a wide aperture, light rays that pass through the dust spot are coming from ALL OVER the lens from millions of different angles (left, right, top, bottom, etc) and there are a million little dark spots which are spread out amongst all of the other light which is hitting the sensor. The lack of light from that one little spot is not noticable. At a narrow aperture (let's get down to an impossible aperture that is one photon wide for the example), you get down ultimately just a few rays of light heading through the aperture, through the dust spot on their way to the sensor. If they are blocked, there IS no other light from other angles to hit the sensor and you end up with a dark spot. It's kind of like the depth-of-field explanation except that the spots are never really in or out of focus. This isn't a perfect explanation. if I could draw it on a piece of paper it would be perfectly understandable. -Charles -- Charles Robinson - charl...@visi.com Minneapolis, MN http://charles.robinsontwins.org http://www.facebook.com/charles.robinson -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
