In a message dated 3/10/2005 12:22:55 PM Pacific Standard Time, [EMAIL PROTECTED] writes: On Mar 10, 2005, at 11:20 AM, [EMAIL PROTECTED] wrote:
> I thought the reason you could spot dirt better was what someone said, > at a > smaller aperture you have a smaller, more concentrated cone of light > than at a > wide aperture. Like those pictures you see with the drawings of the > cone > behind the lens. > > But now that I think about it more, why should more concentrated light > reveal > dirt better? Maybe shorter shadows? So one can pinpoint where it is? > Or is > does that make no sense? You have it right in your first paragraph above ... as I and others have stated, it's a matter of a small aperture acting like a point light source and a large aperture acting like a broad light source. The dust particles are sitting on the filter/lens array some distance from the actual sensor. The imaging of dust particles is a matter of how they cast a shadow under these two light sources. I rigged a little demonstration picture to show how a broad light source and a point light source changes the appearance of shadows, constructed of an olive on a toothpick, a notepad, a plastic bag, and a flashlight: http://homepage.mac.com/godders/shadows.jpg Note how the plastic bag on the left acts light a broad light source, spreading the flashlight beam, and how the shadow on the notebook is indistinct and soft? On the right, the unfettered flashlight beam is a point source and images a hard, sharp shadow. This is exactly what's happening when you see dust at small apertures and no dust at large apertures. Godfrey ======== Thanks, Godfrey, and, what do you know, I wasn't that far off. Marnie :-)
