Resolving Power of Lens.

TM 9-258

corrected lens as in C and D, figure 2-67.

The

gives a flat field of view.

extremities of such a virtual image will be curved away

from the lens so that we call this phenomenon curvature

2-43. Resolving Power of Lens.

of image as in B, figure 2-69.

a. When two points viewed through a lens are so

h. Barrel Distortion. Eccentric rays refracted more

close together that they cannot be distinguished as two

by a convergent lens and cross the axis closer to the

distinct points, these points are not resolved. To make

lens than paraxial rays. Oblique centric pencils also

separation of such points possible, a lens of greater

focus closer to the lens than paraxial pencils. As the real

resolving power is required.

The measure of the

image is formed where the eccentric pencils and the

resolving power of a lens is the limiting angle of

centric pencils come to a focus, it is obvious that such an

resolution which is the angle subtended at the optical

image formed by an uncorrected convergent lens will be

center of the lens by two points which are close together

curved with the extremities of the image close to the lens

and can be just barely distinguished as two distinct

as in A, figure 2-69. This curvature of image is

separate points.

especially troublesome in wide-angle instruments. This

b. It might be supposed that a point of light on the

image on a screen will present barrel distortion (A and B,

object on being refracted through a high quality

fig 2-67) and it will be impossible to focus all the image

compound lens would form a single point of light on the

clearly at one time on the screen. On a screen of the

image. However, a factor termed diffraction tends to

same curvature as that of the image, no distortion or out-

enlarge the point of light so that it merges with another

of-focus effect would appear. The human eye used such

point close to it.

a screen as in C, figure 2-69.

(1) Diffraction causes the image of a point of

i. Correction for Distortion and Curvature of Image.

light to become a tiny disk of light surrounded by a series

Distortion or curvature of image is partially corrected by

of concentric rings of light which rapidly fall off in

employing a compound lens, consisting of one

intensity, a minute bull's-eye target of light. This can be

convergent and one divergent lens, the two having

demonstrated to the best advantage by sharply focusing

different types of distortion. In certain cases, field stops

the lens on a very small single point of light. The image,

or diaphragms may also be used to restrict the passage

if it were greatly magnified, would resemble a series of

of undesired marginal rays. A highly corrected distortion-

concentric rings (fig 2-70). The images of more detailed

free eyepiece or lens system is called orthoscopic; it

objects are made up of these tiny bullseyes.

gives an image in correct normal proportions and

(2) Diffraction sets the final limit to the

sharpness of the image formed by a lens.

2-53