Focusing

TM 9-258

both eyes of an individual, provision must be made to

movement. The two lenses of the eyepiece are mounted

adjust the spacing between the eyepieces of the

in a simple tube and its distance from the reticle or focal

instrument to conform to the interpupillary distance of

point of the objective or erecting system can be adjusted

different observers. The interpupillary distance of such

by rack and pinion, by a simple draw tube or by rotating

instruments generally is adjustable from 58 to 72

the entire eyepiece causing it to screw in or out.

d. On fire-control instruments a graduated scale

millimeters because the eye spacing of nearly everyone

is within this range. If the interpupillary distance of an

generally is provided around the eyepiece. This scale is

The diopter is the unit of

individual is greater than 72 millimeters or less than 58

calibrated in diopters.

millimeters, he is incapable of using instruments of this

measurement of the converging power of lenses (para

type to the best advantage.

10-2). For a normal eye the scale on the eyepiece is set

at zero.

If a positive 2-diopter spectacle lens is

commonly worn, the eyepiece should be set at +2

2-36.

Focusing.

provided the spectacles are removed as they should be.

After carefully focusing the instrument, the reading of the

a. For perfect focusing of a monocular instrument

eyepiece should be memorized and used for future

under all conditions, two adjustments are necessary. If

focusing.

the instrument has a reticle, means must be provided for

e. Low-power telescopes are frequently made

adjusting the distance between the reticle and the

without any means for focusing. Such an instrument is

objective so that there will be no parallax (para 2-46).

termed a fixed-focus telescope. When assembled, such

Also, the distance between the eyepiece and the reticle

an instrument is often so adjusted that light from a

must be adjusted for the eye of the observer. In a

distant source emerging from the instrument, instead of

binocular instrument, it also is necessary to adjust the

being essentially parallel, appears to diverge from a point

instrument to conform to the interpupillary distance of the

40 to 80 inches in front of the eyelens. A telescope so

eyes of the observer.

focused (approximately minus 3/4 to minus 1 diopter) is

b. If the focal length of the objective is short and

more readily adaptable to the eye of the average

the targets in general are at a great distance, the

observer than one focused so that the rays of light are

objective may be adjusted for a target at an infinite

essentially parallel. Because a fixed-focus instrument is

distance when the instrument is assembled in the factory

simple, it can be made entirely waterproof. However, if

and no field adjustment is provided. This is the case with

the power of the telescope is greater than 3. 5 or 4, the

some fire-control instruments and low-power telescopes.

accommodation of the average eye is not sufficient to

c. Focusing eyepieces are commonly found on fire-

permit its use.

control instruments. This adjustment is primarily

designed for focusing the instrument for different eyes

and is referred to as the diopter

Section VII. ABERRATIONS AND OTHER OPTICAL DEFECTS

coma, curvature of image, and distortion. Other factors

2-37.

General.

which may affect the operation of the optical system of

an instrument are resolving power, Newton's rings, light

a. Aberration is a lens or prism imperfection

loss, and parallax.

resulting in an image that is not a true reproduction of the

object.

2-38.

Spherical Aberration.

b. In designing an instrument, correction is usually

made for optical defects with special attention being

a. Light rays refracted through a lens with spherical

given to the use to which the instrument is to be put.

surfaces near its center and those refracted through the

Correction is achieved by using lenses or prisms made

outer portion or margin do not intersect the axis at a

of two or more kinds of glass (called compound lenses or

single point. The outer rays of a convergent lens

compound prisms), and by eliminating rays which would

intersect the axis closer to the lens than the more central

be refracted through the outer edges of lenses (called

ones (fig 2-56) and the opposite is true of a convergent

marginal rays) by equipping the instrument with

lens, considering the imaginary extension of the

diaphragms (field stops) (para 2-38 e), and a suitable

refracted rays (fig 2-57). The result is a blurred image.

eyepiece.

This fault is common to all single lenses with spherical

c. There are six general types of aberrations:

surfaces and is termed spherical aberration.

spherical and chromatic aberrations, astigmatism,

2-42