On Mar 10, 2005, at 11:38 AM, David Zaninovic wrote:
When you're focusing, you're moving the lens body further away from theI know that the effective aperture reduces with the amount of the extension. This smaller effective aperture increases DOF and reduces the light, so why wouldn't it also increase the visibility of dust like real aperture decrease would as it is effectively the same thing.
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.
DISCLAIMER: Of course I could be completely wrong ...
You are completely wrong ... ;-)
"Effective aperture" is a misnomer. The aperture in use does not change, as I said above. The illumination has been reduced by distance, so the "effective aperture" is simply referring to the amount of light that results from the falloff together with the set aperture.
DoF has nothing to do with the imaging of dust on a sensor (or film), nor does this "effective aperture" reduction in illumination ... it's purely a shadow game. See the post I just sent a moment ago with an illustrating picture.
Godfrey
