Figure 2-67. Barrel-shaped (A and B and hourglass (C and D) distortion .

TM 9-258

c. Forms of Distortion. If the curvature of the lines

is formed. This area should be flat in order to form an

undistorted image. When it is not flat, but is concave or

of the image is away from the center of the lens, the

saucer-shaped, the condition is termed curvature of the

distortion is termed barrel-shaped (A and B, fig 2-67). If

image or curvature of the field. An ordinary reading

the lines curve toward the center, the distortion is called

hourglass or pincushion (C and D, fig 2-67).

glass plainly shows curvature of the image (fig 2-68). In

fire-control equipment this defect causes distortion in

d. Curvature of the Image. The field of a lens is the

portions of the field located a distance off the axis.

area in which the image produced by the lens

2-50