Table 1-2.–Amount of light, f/stop, and Exposure Time Relationship
f/stop
1
1.4
2
2.8
4
5.6
8
11
16
2 2
32
4 5
6 4
Relative exposure
Relative amount of light admitted
0.06
0.12
0.25
0.50
1.0
2.0
3.0
8.0
16.0
32.0
64.0
128.0
256.0
The first (lowest) f/stop marked on the lens mount
is the correct value for its largest aperture. The next
number is the nearest f/stop in an arbitrary series that
has been adopted as a standard. In this standard series,
each succeeding number going up the scale (from the
largest opening to the smallest) permits only half as
much light to enter the camera. Thus, as the numbers get
larger, the diaphragm openings (apertures) become
smaller. However, moving the index pointer in the
reverse order, down the scale (from the smallest opening
to the largest), the numbers get smaller and the
diaphragm openings become larger. As shown in
table 1-1, the smallest number may not admit exactly
twice as much light as the next larger number.
Nevertheless, the amount of light admitted remains
inversely proportional to the square of the f/stop, and the
exposure required is always directly proportional to it.
All lenses are indexed with the standard series of
f/stops either completely or in part-except for the first
f/stop (as stated earlier) that is computed to indicate the
correct value of the maximum aperture. The
photographer should become acquainted with this
series, so its relative values are known. The following
table is a listing of the f/stop, better known as the
standard full stops. A comparative exposure based on 1
second at f/4 or 16 seconds at f/16 is also shown (table
l-2).
16
8
4
2
1
1/2
1/4
1/8
1/16
1/32
1/64
1/128
1/256
By studying the table, you can see that when the lens
aperture is opened one full stop, the amount of light
transmitted is twice that of the nearest preceding stop.
And altering the f/stop one full stop less (stopping down)
reduces the amount of light passing through the lens to
one half that of the nearest larger stop.
In summary then:
Light passes through an opening (aperture) of the
lens. The diameter of the aperture can be changed. The
openings are called f/stops. The f/stops indicate to the
photographer that a lens (any lens) with a specific f/stop
allows a given amount of light to the film. Thus a 12-inch
focal-length lens set at f/4.5 gives the same exposure as
a 6-inch focal-length lens set at f/4.5.
The f/stops represent a fraction of the focal length
of the lens for a given lens; that is, an f/4 lens has an
effective maximum opening of one fourth of its focal
length.
From one full f/stop to the next full f/stop, there
is a constant factor of two. As the opening changes from
f/8 to f/l1, the light passing through the lens is reduced
by one half because the larger f/stop (f/11) is a smaller
aperture. When the aperture is changed from f/8 to f/5.6,
the light passed is doubled because the aperture has been
made larger.
1-21
