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A camera works by focusing light rays through a lens onto a piece of plastic film coated with light-sensitive chemicals. The chemical turns dark when light strikes it. The more light, the darker it gets. In the developing lab the film is chemically processed to disable the light sensitivity, so that it can be handled in the light. The result is a photographic negative. When light is shone through the negative onto light-sensitive paper, it produces a normal image.

Photographic film is manufactured in varying degrees of sensitivity. Film that is intended for use in sunlight, for example, is not usually sensitive enough for pictures in dimmer indoor lighting. In that case, the photographer must use a flash to supplement the available light.

Two mechanisms in the camera control how much light falls on the film. There is an aperture, a device very much like the iris in a human eye, which expands or constricts to allow more or less light to pass through to the film. Photographers call the aperture setting the "f-stop". The higher the f-stop number, the narrower the aperture and the less light admitted.

There is also a timer which controls how long the shutter remains open. The longer the shutter interval, the more light falls on the film. Photographers state shutter speeds in fractions of a second, such as 1/250. Here are some examples of what happens when you vary the shutter speed and f-stop.

The technique of manipulating these mechanisms to produce meaningful photographs is called exposure. Since each of the three ways of controlling exposure (film sensitivity, f-stop, shutter speed) has its own side effects, the photographer must decide what's most important. Very sensitive film is usually unable to record fine-grain details. Opening the aperture limits how much of the image will be in focus. Using a long shutter interval may blur the photograph if the subject moves while the shutter is open.

A photograph taken with insufficient light is underexposed, while a photograph taken with too much light is overexposed. Underexposed photographs usually have coarsely-grained variations on a dark color. Overexposed images have patches of white, often bleeding into adjacent areas of the photograph.

Newer cameras have light meters that measure the amount of light entering the lens and computers that calculate the correct exposure, taking into account whether a fast or slow shutter speed is better, or whether a wide or narrow aperture is appropriate. However, even with automatic exposure photographs sometimes come out overexposed or underexposed. This happens when different parts of the picture have different amounts of light on them. The computer reads an average amount of light and sets the exposure accordingly.

For example, if you take a picture of someone who is standing under the porch of a brightly lit house, chances are the house will be correctly exposed but the subject will be a dark silhouette. The computer has no way of knowing that you want your friend in the shadow to be correctly exposed. In this situation you would adjust your exposure to allow more light to enter. The house portion of the photograph will be overexposed, but your friend will look good in the photograph.

The human eye works according to similar principles. It has an iris which opens and closes like a camera aperture. It has a retina which corresponds to photographic film. The human retina changes chemically to adjust adjust sensitivity. There is no feature that corresponds to the shutter speed.

When you go into a dark room after you've just been outside, you have to wait until your irises open and your retinas adjusts chemically. Irises open in seconds, but the retina's chemical changes take a few minutes. Until then, everything looks dark. And conversely when you go back outside, the sun will appear painfully bright until your irises contract and your retinas adjust.

Photography on the lunar surface presents two problems. First, the sunlight is quite bright. The moon is roughly as far away from the sun as the earth, but there is no atmosphere to filter and subdue the sunlight. And along with this is the glare of the sun off the lunar surface.

Second, the difference between light and shadow is more pronounced on the moon since there is no atmosphere to scatter the sunlight and make it more uniform.

Fortunately neither of these problems is unsolvable. A brightly lit scene can be correctly photographed simply by using less sensitive film, and also by closing the aperture and using a faster shutter speed. In extreme cases you can also put a filter (sunglasses for cameras) over the lens to reduce the light entering the lens.

But what about shadow? Well, objects lying in shadow on the lunar surface are not in pitch blackness. Light reflects from space suits, the lunar module, the dust around you. You can open up the aperture and lengthen the shutter speed if the subject you want to capture is in shadow.

A curious feature of photographic film is that it's not necessarily sensitive the same way your eyes are. Manufacturers produce film that is variably sensitive. An extreme example of this was the film invented to photograph nuclear detonations. It is possible to have film that emphasizes subtle differences among darks while representing brights with relative indifference.

In short, the notions that it's too bright on the moon for photography, or that photographic film can't capture both lighted and shaded objects on the moon, are simply not realistic from the photographer's point of view.

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