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
On Mar 10, 2005, at 7:11 AM, David Zaninovic wrote:
Focusing distance changes the aperture on macro lens as you add more extension you are effectively making the aperture smaller.
That is how I understand it.
----- Original Message ----- From: "Juan Buhler" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Wednesday, March 09, 2005 6:22 PM Subject: Re: CCD cleaning
Bingo. That did it, thanks Godfrey, it makes sense to me now. So the aperture matters, but the focusing distance shouldn't.
Thanks!
j
