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| p.2 #13 · silly inverse square law question. |
Curious, you are mistaking things.
Light emited by a constant-output, point light source suffers from spherical divergence (inverse square law) due to the fact that a net ammount of photons are scattered over an increasing-area spheroid, as the distance from the source is increased. In photographic terms, this is incident light.
To a viewer, once an object reflect lights, that light is effectively collimated relative to the spatial position of the viewer. That means that if from the position of the viewer, he is able to register a number of photons, those photons will always be the same, irregardless of how close or far away he is from the subject.
Collimated light does not register an inverse square law decay, because you are always looking at the same ammount of light, no matter how far or near you are from it. 10 photons will always be 10 photons, no matter how far away they have to travel. This is the reason why exposure never changes.
This might seem counterintuitive to photographers, but that is because as we step away, more "background" is allowed into the frame for a constant focal lenght. That background may have a higher or lower brightness, giving the perception of changing illumination of the subject. But as long as you are measuring your subject and not the background, the intensity does not change, ie, your exposure remains constant for the brightness of your desired subject.
A way of thinking of collimated light would be a laser, or a huge wall of with a constant luminance. It dpes not matter how far away or near you get to the wall, the ammount of light it emits is constat. Photographic subjects are not light point sources, even though we do use light point sources to illuminate our subjects.
I totally agree that Collimated light does not register an inverse square law decay. However the light going from the subject to camera is not collimated light. It spreads just like light going from any light source to the subject.
Again lets take the white square on black wall example. Lets assume what you are saying is true and lets say we have 1000 photons reaching our lens from the white square to our lens. Now step back to twice the distance. Now you will agree that the white square is now only taking 1/4th of the area on our sensor, but from what you are saying the sensor is still getting 1000 photons. So that means we now have 1000 photons striking a much smaller area than before. If that was indeed true then the image of the square would become brighter. Which does not happen. So could you please explain how your assumption could still be correct.