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2
0
EXPOSURE-CONTROL FUNDAMENTALS
The two camera exposure-control settings
are lens opening (aperture) and shutter speed.
The size of the aperture determines the amount
of volume of light reaching the film from a
given subject and lighting. The shutter speed
determines the length of time this light acts
upon the film. Apertures are expressed in
f-numbers, which are larger for small openings
and vice versa (e.g., f/15 represents a small
opening, f/2 a large one). Shutter speeds are
expressed in seconds or fractions thereof, wh ich
are generally the reciprocals of the numbers
shown on shutter-speed scales (e.g. 50
=
1/50
sec., and 2
=
1/2 sec.!. At usual apertures, each
f-numbers setting (e.g., f/B) lets in twice as
much light as the next numerically larger one
(f /11) and half as much as the nex t smaller
(f/S.5). Similarly, each shutter speed (e.g., 1/50
sec.! allows light to strike the film twice as long
as the next higher speed (1/12S) and half as
long as the next lower one (1/30). The
interval
between two standard f-numbers (say, f/4 and
f/S.5) or shutter speeds (say, 1 /1S and 1/30)
is
one "stop." Total exposure on the film is
determined by the combination of aperture and
speed. Other things being equal, using the next
smaller f-number (i.e., giving one stop more
exposure) will balance using the next higher
shutter speed (i.e., giving one stop less expo·
sure), and so on. A great range of combinations
(e.g., f/S.5 at 1/30, f/4 at 1/50, f/2.B at 1/12S,
f/2 at 1 /2S0, etc.! will thus yield the same total
exposure. The specific combination you choose
under given lighting conditions will depend
upon the degree to which you want the greater
depth of field (see p. 35) of smaller apertures
and the greate movement-blur preventing abili-
ty of faster speeds (p. 2B).
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