Charles Robinson wrote > Subject: Re: Dust spots ;-( Give me some help! > > 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 > > -- Thanks Charles. I understand that - good enough for me so no need for a picture :-)
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