Figure 2-68. Hourglass distortion in magnification by reading glass .

TM 9-258

e. Formation of Image Curvature. Curvature of the

lens is focused on the enter of the cross, the ends of the

arms are not _ sharp focus and vice versa. This is

image is readily visualized by assuming that a cross is

particularly disadvantageous in a photographic lens

imaged by a lens with this defect (A, fig 2-69). Rays

where a flat plate or film is used. If the image is to be in

from points of light at the ends of the arms of the cross

focus over the entire plate, the image also should be flat.

are brought into focus nearer the lens than rays from the

center of the cross. If the

2-51