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| p.1 #16 · silly inverse square law question. |
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 disseminate iaw with isl. 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.
I think you are misunderstanding what I am saying. I am in agreement that the camera to subject distance does not impact the exposure. The question is only about the exact mechanics of WHY that happens. Light always follows the inverse square law, it doesn't know whether it is coming from a source to a subject or from a subject to another subject. Its the combined effect of light and lens operation which makes the inverse square law irrelevant in determining the exposure of a subject.
Just as someone else said, when the light leaves a point light source, it spreads and as you move the light away or move the subject away, the portion of the light that falls on the subject goes down as per the inverse square law. I believe we are all in agreement on that. The situation is somewhat similar with real life sources which are not point sources. If you have a rectangular light source, you can just think of it as lots of point sources arranged in a rectangle.
Now lets switch over to the other side. When light hits the subject, it gets reflected. That reflected light then gets to our eye or the camera and thats how we see the subject. We are all in agreement on that as well. Now the light that gets reflected from the subject is in no way different in its behavior then the light that gets emitted from the original light source. You can roughly think of the subject as lots of small point sources sending light in all directions. And again the light spreads as it moves away from the subject. A portion of that light falls on the lens. Now since the light is still spreading as it moves forward so the inverse square law still applies. If you move the camera away, the lens will see less of the light. There is just no way to deny this fact. However what happens is that since the camera has been moved away so the lens will now focus this smaller amount of total light on a similarly smaller portion of the sensor. The result is that the light intensity at the sensor does not change, even though the light intensity seen by the lens goes down. A similar effect occurs in the eye as well
I hope that clears things up.