Figure 2-56. Spherical aberration of convergent lens.

TM 9-258

b. The thickness of a lens and its focal length

influence the amount of spherical aberration. Spherical

aberration is least in thin lenses of long focal length.

c. If the convergent lens were ground with

constantly flattening curves to the edge instead of being

ground with each refracting surface as a true portion of a

sphere, the rays would be caused to cross the axis at

nearly a single point and a sharp image would result.

Such lenses, however, would be too difficult and costly to

manufacture and would be correct for only one distance

from the object. Instead, two other methods are utilized

either singly or together which satisfactorily eliminate

spherical aberration (f and g below).

d. Hourglass distortion, barrel distortion, and

curvature of image all result from spherical aberration

(para 2-42). Replacing lenses so they face the wrong

way 'usually will introduce distortion into the system.

This is of interest, therefore, to the instrument repairman.

e. In a lens system such as a complex camera

objective or in a single lens used at a wide angle,

spherical aberration can be reduced at the expense of

Figure 2-56. Spherical aberration of convergent lens.

light intensity by using an aperture stop, field stop, or

diaphragm. The central portion of a lens is most free of

spherical aberration. Tests of a lens will show how much

of the area around the axis may be safely used to form a

sharp image. The practice is to mask out all rays

passing through the lens beyond this circle (A, fig 2-57).

The mask used for this purpose is led an aperture stop.

It is a flat ring or diaphragm of metal or other opaque

material covering the outer portion of the lens. It stops

the rays from entering the margin of the lens but, as it

cuts down the effective size of the lens, it limits the

amount of light passing through the lens. Obviously, in

general, it would be cheaper to manufacture a 4maller

lens rather than to make a large lens and a stop for

masking out marginal rays. Diaphragms or stops have

other uses. See diaphragms or stops in paragraph 4-10.

Figure 2-57. Spherical aberration of divergent lens.

2-43