Figure B-14. Eyelens and fields lens of eyepiece.

TM 9-258

Eyepiece - An optical system used to form a virtual,

is the maximum angle of view that can be seen at one

time. In an instrument, the true field of view is the actual

erect, enlarged image of the real image formed by the

angle of view of the instrument, the maximum angle

objective (fig B-14). The optical system of the eyepiece

subtended at the objective by any objects which can be

usually consists of two lenses, an eyelens and a field

viewed simultaneously. The apparent field of view is the

lens, but the eyepiece may contain another lens (figs 4-5,

4-6 and 4-7).

size of the field of view angle as it appears to the eye; it

Eye relief - See eye distance.

is approximately equal to the magnifying power of the

Eyeshield or eyeguard - A shield of rubber plastic,

instrument times the angle of the true field of view

or metal to protect the eyes of the observer from stray

Field stop - See diaphragm.

light and wind and to maintain proper eye distance.

Far point - The farthest point of clear vision of the

Filters or ray filters - Colored glass disks with plane,

unaccommodated eye. For the normal eye, the far point

parallel surfaces placed in the path of light through the

is infinity. See infinity.

optical system of an instrument to reduce glare and light

Farsightedness - See hypermetropia.

intensity and likewise in observing tracer fire and

Field glass - A type of compact binocular.

detecting outlines of camouflaged objects. They are

provided as separate elements or as integral devices

Field lens - One of the leneses of an eyepiece. It is

mounted so they may be placed in or out of position as

the lens which is nearest the image upon which the

eyepiece is focused. It serves to shorten the eye relief

Finite - Having limits, as opposed to infinite, or

and adds to brightness of the edge of the field (fig B-14).

without limits.

Field of view - The open or visible space or angle

Fire control - The determination and regulation of

commanded by the eye. It

the direction of gunfire. More specifically, it refers

B-14