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
