and use a 200mm lens. The fence at this distance, with
the 200mm lens, is the same size as it was at 10 feet with
the 50mm lens. The cow is now 140 feet from the
camera, but her image is four times larger. In the
photograph, it looks as if she were only 35 feet away or
25 feet behind the fence. The results! An interesting
picture and pleasing composition. Choosing viewpoint
and then selecting focal length for image size is one of
the most important functions you should consider when
selecting lens focal length.
HOW TO USE LENSES
Today, the Navy photographer is applying
photography to the ever-widening specialized and
technical fields within the modem Navy. This has led to
greater emphasis on the correct and accurate use of the
most important part of the camera-the lens. The higher
standards of picture quality and the greater interest in
picture taking regardless of lighting conditions, all
demand more attention to the correct use of lenses. No
matter how good the quality of the lenses, if
photographers do not use them correctly, they will not
do us or the Navy any good.
f/stop of a Lens
To use lenses correctly, the photographer must
understand the relationship between the aperture of a
lens and the brightness of the image produced at the
focal plane. The aperture of a lens is simply the opening
through which light passes. The aperture is controlled
by an adjustable diaphragm or iris. Each setting of the
diaphragm is called an f/stop and is always read as a
number, not as a fraction or true ratio. It is referred to as
the f/stop or the f/stop of the diaphragm opening. This
value is designated by a lowercase f with a slant (/)
between the f and the value. For example, f/8 means that
the diameter of the opening in the diaphragm is one
eighth of the lens focal length, but only "when the lens
is focused on infinity." In this example f/8 is the
effective aperture. If the lens were focused at other than
infinity, f/8 would then be the relative aperture. In the
study of the relationship between aperture and image
brightness, the term relative aperture is used frequently.
The term relative aperture then refers to the ratio
between the effective aperture of the lens and its focal
length. The relative aperture of a lens is controlled by
two factors: (1) the diameter of the beam of light passed
by the lens; and (2) the focal length of the lens, which
governs the size of the area over which the light is
spread.
f/stop Applications
The formula to determine the f/stop of a lens is as
follows:
Where:
F = focal length
D = diameter of the effective aperture
f
= f/stop, or the relative aperture
EXAMPLE: To find the f/stop of a lens that has a focal
length of 8 inches and the diameter of the effective
aperture is 2 inches, use the formula below.
Therefore, the lens has a relative aperture of f/4.
When the diameter of the opening (aperture) of the
lens is made smaller, less light is admitted and the image
formed by the beam of light passing through the smaller
opening becomes dim. As the size of the opening is
reduced, the ratio between the aperture and the focal
length increases. Thus an inverse relationship exists
between the E/number and the relative aperture; as the
f/stop becomes larger, the size of the relative aperture
decreases.
Since the f/stop is a ratio of focal length to the lens
diameter, all lenses with the same f/stops regardless of
focal length provide the same amount of light on the
focal plane; that is, when all the other factors that affect
image brightness remain constant (fig. 1-23).
DIAPHRAGM
There is in every lens assembly a mechanical device
for controlling the amount of light that passes through
the lens. This mechanism may have a fixed size, or it
may be designed to provide a selection among a number
of sizes that can be given to the aperture in a lens. This
device is a diaphragm, and its scale increments are
called f/stops (fig. 1-24). It is located within the lens to
cut off or obstruct the marginal light rays while
permitting the more central rays to pass. Most lenses
have a series of thin metal leaves for this purpose. These
leaves are arranged and shaped to provide an
approximately circular opening that can be changed in
1-18

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