Eye Distance or Eye Relief.

TM 9-258

the ratio of focal length to lens diameter of aperture. It is

termed the eye distance or eye relief (fig 2-55). The eye

also known as the speed of a camera lens and is written

must be placed at this distance from a collective

as f/16 (for example) or f/3. 5. At f/16 the lens is being

eyepiece in order to see most effectively through the

used at an aperture (opening) of 1/16 of its focal length

instrument.

and at f/3. 5 the lens is being used at an aperture of 1/3.

5 of its focal length. As the denominator increases, the

focus becomes sharper (lessened aberration) but the

light and brightness of image decrease as the aperture is

smaller. The time of exposure, therefore, must be

lengthened proportionally.

As the denominator

decreases, the lens is said to become faster (wider

aperture or lens opening) and it can be used at faster

shutter speeds (shorter exposure time). If it is desired to

photograph action, a fast lens must be used; otherwise,

the picture will be blurred from the motion.

2-34. Eye Distance or Eye Relief.

a. The exit pupil (fig. 2-54) is the term applied to

the rays of light which emerge from any optical

instrument with a collective or convergent eyepiece and

form the image that is seen by the eye. The exit pupil of

such an instrument can be seen as a brightly illuminated

disk floating in space if the instrument is directed toward

an illuminated area and its eyepiece is held about 15

Figure 2-55. Eye distance.

inches from the eye. The position of the exit pupil can be

fixed on a screen of translucent paper or on a plate of

c. If the eye distance of an instrument is so limited

ground glass. In this manner, the distance of the exit

that the exit pupil cannot enter the eye pupil or if the

pupil from the last glass surface of the eyepiece can be

eyelashes touch the eyelens, observation through the

Figure 2-54. Exit pupil.

measured.

instrument will become tiresome and the instrument

cannot be used most effectively. When the instrument is

to be mounted on a weapon, if the eye distance is not

considerable, recoil, of the gun will make observation

dangerous.

d. In the design of the instrument, the proper

location of the exit pupil must be given most careful

consideration. If it is too far back, the eyepiece will

become too cumbersome in design; if it is placed too

close, the user of the instrument will suffer discomfort

and will be unable to use the instrument to the best

advantage.

Proper eye distance depends on the

purpose for which the instrument is to be used

(para 5-27).

2-35. Interpupillary Distance.

a. The spacing between the pupils of the eyes

make stereovision possible because each eye views an

object from a slightly different viewpoint. From the fused

image gained from the two viewpoints, the observer

receives the impression of depth. This spacing of the

b. The full field of view of a telescope can be seen

eye pupils varies to a considerable degree in different

only when the pupil of the eye of the observer is at the

individuals and is known as Interpupillary distance. This

same position as the exit pupil. The distance from the

distance is measured in millimeters by the

eyelens to the exit pupil is

interpupillometer M1.

b. In the designing of instruments to be used by

2-41