I'm sure there is some physics explanation. But in my simple view,
a subject's brightness is not dependent on the location of the viewer.
The viewer does not have any effect on what is viewed (quantum
mechanics and psychic phenomenon aside, and with deference to
Schrodenger's Cat). Whereas if I move a light source around, what
it illuminates will change. But the viewed subject's illumination is only
affected by the light that illuminates it, not by the camera or viewer.
The only difference is for light that reflects rather than emits from
the surface of the subject, for which we photogs have polarizers.
But even that will not matter as long as we remain at the same
viewing angle to the subject.
I think all the confusion is thinking the light from a subject
is illuminating the camera. It is true that the amount
of light diminishes with distance, but so does the subject size.
All this has been said already.
If a spot meter is aimed at an illuminated subject, the reading will
not depend on distance, as long as the metered area is smaller
than the subject size, simply because the subject's illumination is
not dependent on the viewer. If you use an average meter, then
it will show a decrease in overall scene brightness, unless there
are other illuminated objects.
If you call the subject a point source, then there is no meter available
that can be that small, but if there were, the reading would be
independent of distance on a very clear day.
Another way to think about it. If you shoot with a 50mm lens
at F/2, then move further away, but shoot with 100mm at F/2
such that the scene area is the same, the exposure will be the same.
And the 100mm F/2 will be a larger opening so it can capture the
same amount of light as the 50mm F/2. The F number being related
to the focal length and lens opening.
WayneS - pseudo physicist
At 11:04 PM 1/4/2009, you wrote:
>I accept what you're saying but it is not an explanation in a physics sense.
>
>Chuck Norcutt
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