In a message dated 3/10/2005 10:51:35 AM Pacific Standard Time, [EMAIL PROTECTED] writes: Darned if I understand what relevance that has to anything in this question thread, David.
When you're focusing, you're moving the lens body further away from the sensor/film plane. The nodal point is essentially an illuminating body from this point of view, and light falls off at a rate equal to the square of the distance (Inverse Square Law). The closer you focus, the further away from the sensor/film plane that illuminating body is. At some point with a macro lens, the light falloff due to extension becomes significant and you have to account for it in your exposure. Nothing about the aperture changes, unless the macro lens has some sort of auto-compensating diaphragm mechanism: the light is simply becoming less bright through distance. What this has to do with the imaging of dust at small and wide lens openings is a mystery. Godfrey ======== I am now more confused than when we started. Of course, I am don't get the macro stuff at all. So maybe that is what I am not getting. That, however, I can let go. 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? Lepp just explained how to do it (focusing blank wall, f22, etc.) and when you'd see it while shooting (smaller f stops/sky) -- I wish he explained why it worked that way. Marnie :-(
