Exposing film is simple; just let light strike the film.
photography, this is rarely useful.
First, you usually want to record an
image. Second, you want to record an image of varying brightnesses. (After
all, if the brightness was the same everywhere, the image would be
uninteresting - just a splotch of black, gray, or white.) So the problem of
exposure becomes one of "how much?".
Too little exposure, and important
parts of the image do not receive enough light to be recorded; too much and
the film is exposed until it is as dark as it can get and the image is
eventually lost in a totally black negative.
The problem of "how much?"
involves several things, such as the brightness of the light, the type of
film used, and the reflective characteristics of the objects in the scene.
But before investigating all these factors, let's first look at ways to
control exposure, then at ways to find the correct exposure.
As you read earlier, when more light strikes the
film, the resulting image becomes darker. The problem then becomes one of
finding out how much light will make the image neither too dark or too
light. The correct amount of light must be accurately measured.
a. There are two requirements in measuring light; you must control the
intensity (brightness) and the amount of time light strikes the film. Think
of it as filling a bucket with water.
The amount of water which finally
winds up in the bucket depends on both how fast the water was running and
how long it was allowed to flow before it was shut off. If you want to fill
the bucket to a precise depth, you can do so by running the water rapidly
for a short time, or by running it more slowly for a proportionally longer
Either way, the bucket will have the same amount of water in it.
When making an exposure, you want to "fill up" the negative until it is
full, but no more. Just as overfilling a bucket won't make the water get
any deeper (because it merely spills over the edge and runs away), adding
more light than needed to make the negative turn black is also a waste. The
negative won't get any blacker. In photography, the rate of flow of light
is called intensity, and time is the length of time light is allowed to flow
(fig 1-2). So the formula reads:
The principle of equivalent exposure