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However at the same time the subject is becoming smaller in your frame, i.e. taking up less pixels. Thus the amount of light falling per pixel remains the same. So the exposure remains the same.
According to this, you're implying an offsetting change predicated upon camera to subject and image size. A subsequent corollary would be that if I move further away and change lenses to retain (or enlarge) the subject size in the frame (no longer relatively offsetting as you've proposed), it would REQUIRE a change in exposure.
Here again, this is not true. Whether the subject is filling the frame 100%, 10% or 1% has no bearing on the illumination / exposure for the subject. It is true that your camera's average reflected meter reading might change as a byproduct of filling/reducing your frame with more/less brighter/darker area for metering the entire scene... but that does not change the illumination/exposure for the subject.
Whether you use a 50mm lens to photograph the moon, or a 500mm lens (changing the size of the subject in the frame) doesn't change the illumination, nor the appropriate exposure. Granted, you'll get a different reflected meter reading due to the size of the subject change ... but that doesn't change the exposure.
It isn't because BOTH are simultaneously changing that the exposure remains the same ... it's because NEITHER are changing when you change camera to subject distance. If the size of the subject in the frame were to impose a required exposure change, then the exposure for a 4X zoom (i.e. 24-105) would not be able to remain the same for a wide angle view of the subject as it is for a telephoto view of the subject.
Camera to subject distance is not responsible for illumination / exposure changes.
Consider this ... while a lighthouse cannot illuminate a ship that is miles away (inverse law renders the illumination too low), those on the ship can readily see (or photograph) the light emitting from the lighthouse ... i.e. unaffected by the inverse square law). In fact, a lit cigarette on a moonless night can be seen from miles away, yet it can essentially illuminate nothing beyond the person holding it.
How light is seen at the subject is not the same as how it disseminates from the subject. If the inverse law applied as you are espousing, the lights of a night skyline would not be able to be seen nor photographed from such distances neither.
BTW ... if camera to subject distance is responsible for impacting exposure ... how much exposure difference would Neil Armstrong (standing on the moon with camera a scant few feet away) have to use in order to account for the difference in camera to subject distance than we (standing on earth @ 240,000 miles away) have for exposure values of the moon illuminated by the sun with the same light to subject distance? Given the orders of magnitude that Neil Armstrong was closer to his subject than we are ... a camera didn't exist that would accommodate the inverse law relative to camera / subject distance variance from the exposure values we see from here ... IF camera to subject distance were part of the exposure determination.
Coming back down to earth ... bouncing lite off of a white reflector panel ... the light that is reflected off the panel intended to illuminate a different subject will appear to further disseminate iaw with isl, but that is because it is simply replicating dispersion through AI=AR over the angles/area involved @ the reflector. But. if you are photographing that white panel, you are photographing the amount of light present at the panel, not the subsequent dissemination of the light that is traveling away from the panel ... and your exposure for the amount of light illuminating the panel doesn't doesn't change with camera to panel distance.
Camera to subject distance is NOT responsible for illumination / exposure values and does NOT change with the inverse square law with regard to viewing / photographing.
Edited on Nov 03, 2012 at 12:27 PM · View previous versions