2-16. RESOLVING POWER.
a. The resolving power of a lens, also called the definition of a lens,
is its ability to reproduce fine lines. Resolving power is usually measured in
lines per millimeter. Thus, if the resolving power of a lens is 100 lines per
millimeter, the lens records 100 definitely separated lines per millimeter.
Most high quality lenses have a resolving power that far exceeds the resolving
power of film.
b. Two factors that influence the definition of a lens are the quality of
the lens and diffraction.
The maximum definition of any lens depends on the
minimum circle of confusion.
This, in turn, is controlled by the quality of
the lens. Geometric limitations in lens design make it impossible to create a
lens of uniform quality from center to edge.
c. The
edges
retain
aberrations
that
greatly
affect
definition.
Accordingly, most manufacturers recommend use of the optimum aperture of a
particular lens. This refers to the aperture at which the lens operates best.
The optimum aperture is normally two to two and a half stops above the maximum
aperture of the lens.
If you have a lens that is rated at f/2, the optimum
aperture would probably be between f/4 and f/5.6.
2-17. DEPTH OF FIELD.
a. Depth of field is the distance from the nearest point of acceptable
sharp focus to the farthest point of acceptable sharp focus of a subject being
photographed, (fig 2-23).
105
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