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
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
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
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.