On Thu, 05 Sep 2002 03:12:40 GMT, "Kevin Neilson" wrote:

>I was looking for a graduated neutral density filter, and it occurred to me
>that I don't understand how these work. It seems obvious at first glance;
>the upper part of the filter is darker, so it will make the sky darker. But
>that assumes that the photons from the sky only go through the top part of
>the lens, which I don't think is true at all. A sky photon hitting any part
>of the outer lens will be refracted so it ends up in the correct spot on the
>film (if it is in focus). I even put a card over the upper half of my lens
>and looked through the viewfinder, and I noticed an overall darkening of the
>scene, but the upper part didn't seem any darker than the bottom. So how
>does this work?

The more the lens is stopped down (use your DOF preview
control, if your camera has one - otherwise you will be
viewing only with the lens diaphragm open...), the filter
is more and more brought toward the DOF range of the lens,
beginning to image the two halves enough to have an effect
(the effect will be strongest on a WA focused close and
at a small stop, weakest on a tele, focused near infinity, and
used at a wide stop...).