NOTE:
When working these problems you will find it easier to convert
scale to a decimal; e.g., or 1:2 becomes .5, 1/3 or 1:3 becomes .33,
and or 1:4 becomes .25.
2-15. FOCUSING LENS FOR ONE OBJECT.
a. When light rays from a far object pass through a lens, they form a
sharp image close to the lens. When light rays from a near object pass through
a lens, they form a sharp image farther from the lens.
This means that the
lens must be focused on either the far or the near object, depending on which
one the photographer wants to be sharp. If a sharp image of the near object is
desired, the lens should be focused by moving it farther away from the ground
glass or film. If a sharp image of the far object is desired, the lens must be
moved closer to the ground glass or film.
b. Figure 2-22 illustrates this process.
Light coming from far object,
A, passes through the lens to form a sharp object at A1, and light coming from
the nearer object, C, passes through the lens to form a sharp image at C1.
Figure 2-22.
Focusing for one object.
c. To obtain a sharp image of object A, move the film to A1. To obtain a
sharp image of object C, move the film to C1. This process of moving the focal
or film plane of a camera to the point of intersection of a given ray is called
focusing.
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