Minolta X-700 - Manual (Page 31)

When you take a picture, light from the subject
passes through the lens and open shutter, striking
the f i l m to form an image. To obtain correct
exposure for the subject's brightness and f i l m being
used, the aperture (size of the diaphragm opening)
and shutter speed (length of time the shutter
curtain is kept open) must be controlled.
As indicated by the aperture diagram next to
each f-number in the figure, large f-numbers (e.g.,
f/16 and f/8) represent small apertures, and small
f-numbers (e.g., f/2 and f/1.4) represent large
apertures. Each standard f-number setting (e.g.,
f/8) lets in twice as much light as the next
numerically larger one (f/11) and half as much as
the next smaller one (f/5.6). This difference in
exposure between standard f-numbers is called one
" s t o p " .
Shutter speeds are expressed in fractions of a
second (generally the reciprocals of numbers
shown on shutter-speed scales) and in seconds.
Each standard shutter speed (e.g., 1/60 sec.) allows
light to strike the film twice as long as the next
faster one (1/125) and half as long as the next
slower one (1/30). This difference between stand-
ard shutter speeds is also called one " s t o p " .
Total exposure on the f i l m is determined by the
combination of aperture and speed. Using the next
smaller f-number (i.e., giving one stop more expo-
sure) will balance using the next faster shutter
speed (i.e., giving one stop less exposure), and so
on. A great range of combinations (e.g., f/5.6 at
1 /30, f/4 at 1 /60, f/2.8 at 1 /125, etc., all of which
fall on the same diagonal line) will thus yield the
same total exposure.
The diagonal lines correspond to exposure values
(EV); all of the aperture/shutter-speed combina-
tions indicated by a given line will produce the
same exposure. At any specific film speed, the EV
increases by one each time the subject brightness
doubles, and thus the required exposure will
decrease by one stop. On the other hand, when the
EV is one unit lower (i.e., when the subject is only
half as bright), the exposure must be increased one
stop.
The film-speed-coupled metering system of the
camera measures the brightness of the subject and
computes the EV needed for proper exposure,
which is then used for setting the combination of
aperture and shutter speed.
2 9