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Re: silly inverse square law question.

RustyBug wrote:
curious80 wrote: So when we go back we do not get any more light rays because everything outside the original area is black.


We get the same number of light rays reflecting off the black (unless the black is a black hole that will not allow any light to escape/reflect).

We get the same number of light rays, but the additional amount of energy being absorbed by the black cloth, renders less energy being reflected. The magnitude of the energy is reduced, but the direction remains iaw AI=AR. Thus the amount of energy reaching the sensor is now less and and cannot be converted into as strong of a signal (or is insufficient energy to effect a change to the negative), and that area of the scene is then recorded as black. The original area still yields a same exposure value, because it is the same reflection of energy with or without a black cloth on surrounding areas.

AI=AR (absorption & refraction).

Conservation of energy requires that the total amount of energy remain constant and is divided among that which is absorbed (color), refracted (transmitted through translucent/transparent material) and reflected. As the refractive index, angle of incidence and color absorption characteristics change, so does the amount of energy remaining available for reflection.

Sure, to be completely pedantic we have to say that the black part does reflect rays which are very low energy and these form the very dark image on the sensor (though it doesn't effect the end result). So to summarize with this assumption: Initially we set the camera such that we capture just the original uncovered part of the box as I drew above. Now we move the lens+sensor back. We loose some rays from the original part of the box, but as you said we gain some more "low energy rays" from the black part of the image. However these extra low energy rays are just used to form the dark image of the black cloth. As far as the image of the original box is concerned, it only depends on the rays from come from that original part of the box. And we have lost some of those rays. So the total energy that is not reaching from original part of the box to the image of that original part on the sensor has gone down. Are we in agreement now?

Nov 07, 2012 at 05:37 AM

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