Figure 2-53. Comparison of light entering eye with 2-power telescope.

TM 9-258

c. An apparent field of view of approximately 450

2-33. Brightness of Image.

may be considered as a practical maximum for a highly

a. Brightness of Image and Exit Pupil. The more

corrected eyepiece. Thirty--five or 400 is a more

light that can be brought from each point of the object to

common value and in the more simply constructed

the eye, the brighter will be the image that is formed.

eyepieces the field may be only 250 or 300. This

This characteristic is known as brightness of image. The

apparent field of view, divided by the power of the

point where the image is brought to the eye is called the

eyepiece, determines the maximum value of the true

exit pupil of the instrument.

field for any given magnification. For example, if an

b. Magnification and Brightness. When one looks

eyepiece corrected for an apparent field of 400 is used

at very distant objects with the naked eye, almost parallel

and a telescope of 10-power is required, the maximum

rays of light enter the pupil of the eye and form an image

true field of the instrument would be only 4.

of a certain size on the retina. When one uses a

d. Other factors can further limit this maximum true

telescope of 2-power magnification, the light forms an

field. The components of the erecting system of the

image on the retina which is twice as high and twice as

instrument might not be sufficiently large to transmit the

wide or an image that covers four times the area of the

full useful area of the image and would decrease the

image that would be formed by the naked eye of the

field. Some aperture in the instrument might be so small

same parts of the objects (fig 2-53).

as to restrict the field.

Figure 2-53. Comparison of light entering eye with 2-power telescope.

c. Loss of Light. There is always a considerable

diameter is from 1/4 to 3/10 inch. A telescope or other

instrument for use at night should have an exit pupil

loss of light by absorption and by reflections at the

aperture (fig. 2-54) of this size. The aperture of the

surfaces of the lenses. While this loss may be as great

objective must then be sufficiently large (1/4 to 3/10 inch

as 75 percent, every effort is made to reduce the loss to

times the magnification of the instrument) to insure that

a minimum (para 4-13).

d. Objective Aperture. A contributing factor in

the pencil of light will fill the pupil of the eye. During the

daytime, the useful aperture of the objective of such an

brightness of image is an objective lens aperture (usable

instrument is correspondingly smaller.

portion) large enough to permit the eyelens to produce

f. Sacrificing Brightness of Image. Some of the

an emergent beam that will fill the pupil of the eye. On

telescopes used for military fire control purposes are

the other hand, enlarging the objective beyond this point

made with objective smaller than that indicated by/this

affords no greater brilliance because additional light is

formula. Brightness of image has been sacrificed to

prevented by the iris from entering the eye.

e. Exit Pupil Aperture. During the day, the pupil of

obtain a more compact instrument.

g. Aperture Ratio of Camera Lens. This is

the eye is from 1/10 to 2/10 inch in diameter. At night,

the pupil may dilate until its

2-40