Figure 2-63. Proper refraction of light .

TM 9-258

c. A lens with spherical or plane faces properly

vertical focal plane; then they become broader ovals until

they are circular; and then they again become a line in

ground will not show astigmatism for points near the axis

the horizontal focal plane at right angles to the first line

but will show astigmatism for points laying at a

(fig 2-64). Between the two focal planes is an area

considerable distance from the axis. The face of the

known as the circle of least confusion. It is in this plane

lens is then presented obliquely to the incoming light

that the most satisfactory image is secured.

rays. Cross sections of the cone of light refracted by the

lens become successively narrow ovals until they

become a line in the

2-47