2009/1/4 Ken Norton <ken@xxxxxxxxxxx>:
> What changes with distance is subject size. This follows the same
> square-root laws as light fall-off. So, if you consider that at 4 feet a
> subject fills the viewfinder, it is at, say 100%. But at 8 feet, the
> subject only occupies 50% of the viewfinder. The quantity of light reaching
> the viewfinder is 50%, but that which does is still at 100% intensity.
Right answer, but I think your maths is a bit out - moving from 4 feet
to 8 feet, the image fills 25% of the viewfinder (it's an area, not a
length) and the light from the subject has decreased by a factor of 4.
So 1/4 of the light gets focused onto 1/4 of the area, hence same
brightness
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