Once your choice of film is settled, the basic condition of exposure is settled with it. You are now left with the
problem of scaling the light you find in front of your camera to the amount your film needs.
Your job is to judge the light reflected from the subject you are about to photograph. Your grandfather as an
amateur photographer used to take into account his geographical position, the time of the year, the hour of the
day, the state of the sky as well as the tone of the subject itself, and by so adding one thing to another size up
the light reflected from the subject. The experienced professional, of course, hardly ever worked that way. He
just had a look and he knew.
Today a light meter or exposure meter does the same for any photographer. It takes a look, it measures the
light and it lets you know.
In fact, it does more than that. It translates the light measured straight into terms of photographic exposure. It
does so by presenting you with the choice of aperture numbers and shutter speeds, sorting them out in pairs.
Aperture and Speed
The aperture number or f-stop controls the amount of light allowed to enter through the lens. These numbers
run in a series: 2-2.8-4-5.6-8-11-16-22. Each higher stop number lets through half the light of the next lower
number (next larger stop).
The shutter speed controls the length of time for which the lens is kept open to light. Shutter speed figures
represent fractions of a second: 2 = ½ second, 4 = ¼ second...500 = 1/500 second.
The actual exposure is a product of these two: "how much" and "how long". A large amount of light striking the
film for a short time may produce an image similar to that produced by a small amount of light striking the film
for a long time. Hence the free choice from a series of balanced aperture-shutter combinations offered by your
exposure meter: more or less open apertures paired with more or less quick shutter speeds, and more or less
stopped down apertures paired with more or less slow shutter speeds.
Cameras of recent vintage combine these pairs, made up of aperture stops and shutter speeds, into single
figures which are then called exposure values or light values. Once you set the exposure value suggested by the
exposure meter both the aperture and the shutter speed move up and down in step against each other, and so
keep the resulting exposure right at every combination.
With exposure meters built into the camera and coupled to cross-linked aperture-speed controls you may even
be able to set the right exposure without looking up the number either on the meter or the camera. You set the
exposure visually by following the swing of the needle in the meter and matching it with a pointer or a circle on
the spot where the needle comes to rest.
Choosing the Combination
But whether you work out the right exposure from an elaborate table or chart;
whether you are presented with a series of exposure values or pairs of aperture figures and shutter speeds;
whether you just set the exposure to the point shown by the needle in your meter;
or whether the meter does the job for you altogether -- you still have one decision to face: which aperture-
shutter combination to choose for any given shot. Paradoxically enough, they all are right yet one is better than
the other.
Why should it be so?
Because both the aperture and the shutter also have secondary functions and effects.
The aperture not only controls the amount of light that is allowed to pass the lens -- it also has a bearing on
how much of the image will be sharp.
The shutter, in controlling the length of time for which the light strikes the film, will inevitably record any
movement that happened during that time as a slight or greater blur.
So you are left with three things to think of:
How fast is the action you want to catch?
How much of the scene in front of the lens has to be sharp?
Is the light good enough to go by either?
If there is fast action you have to choose and pre-set an appropriately fast shutter speed (p.38) and then pair it
with the stop you get by the cross-coupled controls or from your meter.