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| p.4 #8 · silly inverse square law question. |
curious80 wrote: When you move away from the subject, the light reaching from the subject to the camera of course goes down as per the inverse square law.
And then this:
By way of example, if I take spot readings of a gray target as I move back, and taking the light back with me, the readings change in accordance with the inverse square law; but if I leave the light stationary and move only the spot meter, the readings remain constant.
A question to curious80, if I may :
an evenly lit flat e.g. board, metered with a 5° spotmeter at varying distances, achieves identical exposure values. How comes?
A spotmeter meters reflected light from a specific (5°) area using the same lens and using the same sensor area each time. It will give you the same exposure value of the subject based on its reflected light reading design disregarding distance. This reflected reading is not affected by ISL.
Without going very far from the subject, (and assuming the subject isn't very large compared to the measuring distance) the use of an incident meter pointed towards the subject (now a pseudo light source as curious80 seemed to be first inferring), at the same varying distances will illustrate to you that the ISL is still (only approximately) in effect.
Subjects and light sources are being mixed up it seems. Confusing matters in an under current is then the role of apertures, focal length and magnification !
if you light a subject with off camera light source, but move the camera away from the light and subject will this effect exposure to the same degree as if you moved the light farther from the subject?
L2/S2 <-20 feet-> L1/S1 <-10 feet-> camera.
(light source and subject #2 is 30 feet from the camera, light source and subject #1 is 10 feet from camera.)
It doesn't matter how far you are from your subject with your camera, only the subject to light source distance and the exposure value it provides is important.