> From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
>
> A fixed quantity of light is expanding to
> fill an ever larger virtual sphere whose surface area is increasing
> according to the square of the radius (4*pi*r^2). What I don't
> understand is why, once the light has struck and reflected off the
> subject, why the subject is not treated as the "light source" for the
> reflected light that's emanating from it in the same sense as the
> sun is
> a light source.
I think the physics should be the same -- it's called "reciprocity:" a
source of energy transmitted through a medium behaves exactly like a
receiving energy sink at the same distance.
What's different is focal length. If you have a zoom lens on your
light source, you can defeat the square law by putting the same number
of photons in a smaller cone. (Those of us who have constructed
fresnel lens "hoods" for our flashes have experienced this.)
If you're shooting a scene from 10 metres that fills your viewfinder,
then you move to 20 metres and zoom in to capture the same scene,
you're capturing the same number of photons.
:::: The claim of being able to supply all the gas and oil we need if
the price goes high enough is hokuspokus. No price on earth is going
to create oil that isn't there. -- M. King Hubbert, 1976 ::::
:::: Jan Steinman <http://www.EcoReality.org> ::::
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