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It's common to think that pictures of the moon require unusual exposures. It's a nighttime shot, so naturally one expects to need fast film, fast lenses and long exposures. But this is actually not the case at all. Photographing the moon is like taking pictures at the beach! A shot at the beach at noon usually requires you to stop your lens way down, use the fastest shutter speed you have, and squint through the too-bright viewfinder. Think about it for a moment. The beach we are photographing is illuminated by the sun, nearly overhead at noon. The sand reflects the incident light fairly evenly into all directions. The camera intercepts this reflected light and is adjusted so that the right amount of it exposes the film.
Well the moon is really not much farther away from the sun than the beach is, so the illumination level is about the same (the Apollo astronauts used exposures that were typical of terrestial photographs). The moon's surface is not all that different from a sandy beach, so the light is reflected off into all directions in about the same way. The amount intercepted by the camera will be the same. And so the exposure will be about the same: use fast shutter speeds and small apertures. But what about the fact that the moon is so far away? A quarter million miles seems like it should attennuate the light quite a bit... and what about that inverse square law anyway? The inverse square law still holds, but what the film is sensitive to is the amount of light per unit area of film. When you take a picture of the beach, there is a certain amount of light coming from each grain of sand. If you take your picture from a few feet away you get the light from all the sand that fits into the frame. If you step back, the light from each grain falls off (according to the inverse square law: at twice the distance you get one-fourth the light), but now you can fit more sand into the frame! In fact if you step back to twice the distance you will be photographing 4X the amount of sand, exactly the amount to make up for the falloff of light from each grain. So the amount of light per unit area of film stays the same regardless of whether you are taking closeups, or distant shots of the beach. And the proper film exposure remains the same. This also holds for the moon; you are just taking a really long-distance picture of a lunar beach! Like most people, I was a bit surprised when I first encountered this. I think this explains why many beginner's shots of the moon are almost always washed out. I know I found it a bit annoying when the recommended exposure for my first lunar eclipse sequence required a shutter speed I could not reach (1/4000 sec)! As elegant as the beach analogy seems, it does NOT hold true for other planets. They are much farther (or closer) from the sun than the Earth and moon and so the incident light is considerably different. It is also not useful for taking pictures of stars, comets, or just about anything else in the sky. But it is a good guideline for the moon. I used Fred Espenak's Lunar Eclipse Photography guidelines with success (but then I also strongly bracket my exposures, wanting to insure that some frame in the range will turn out).
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