Diopter movement

TM 9-258

Dispersive lens - See diverging lens.

a power of 4 diopters. The lens with the shorter focal

Distortion - The aberration of a lens or lens system

length has the greater power in diopters. See prism

diopter.

which causes objects to appear misshapen or deformed

Diopter movement - A term applied to adjustment of

when seen through the lens or lens system. This defect

the eyepiece of an instrument to provide accommodation

is known as "barrel-shaped" distortion when the center of

for eyesight variations of individual observers.

the field of view is enlarged with respect to the edges

Diopter scale - A scale usually found on the

and "hourglass" or "pincushion" distortion when the

focusing nut of the eyepiece of an optical instrument. It

edges are enlarged with respect to the center (fig 2-67).

measures the change in refracting power of the eyepiece

Wavy lines are sometimes produced by improperly

in diopters to introduce a correction to compensate for

shaped or polished windows, prisms, and mirrors. When

the-nearsightedness or farsightedness of the individual

caused by a lens, it is due to varying magnification of

observer. It permits presetting of the instrument if the

different parts of the image.

observer knows his diopter correction.: Dioptometer -

Diverge - In a lens, to deviate the light outward from

Precision instrument used in determining the diopter

a common center in different directions. As applied to

setting of another instrument.

the eyes, the action of the pupils being directed outward

Dioplopia - See double vision.

(Walleye vision) or in not being brought to a common

Discernible difference of convergence angles - The

focus.

Divergent lens - See diverging lens.

differences in the angles of view from the two eyes to

Diverging lens - Also known as divergent lens,

objects or parts of objects. These differences, although

negative lens, concave lens, dispersive lens. A lens

small, permit persons with normal, two-eyed vision to

which causes parallel light rays to spread out. One

distinguish which of two objects is farther away, making

surface of a diverging lens may be concavely spherical

stereoscopic vision possible without resorting to the other

factors which aid depth perception.

and the other plane (planoconcave), both may be

Dispersion - In optics, the separation of a beam of

concave (double concave) or one surface may be

concave and the other convex (concavo-convex

white light into its component colors as in passing

meniscus diverging). The diverging lens is always

through a prism (fig 2-51). See spectrum.

thicker at the edge than at the center (fig B-11).

Figure B-11. Diverging lenses.

Diverging meniscus lens - See meniscus diverging

Double right-angle Abbe prism - See Abbe prism.

Double vision -

lens.

Double concave lens - A lens with two concave

a. A malfunction of a binocular instrument

surfaces (fig 4-1).

causing two images to be seen separately instead of

Double convex lens - A lens with two convex

being fused. It is caused by the optical axes of the

surfaces (fig 4-1).

B-11