Figure 2-65. Coma

TM 9-258

Figure 2-65. Coma

d. The total image of the point offset from the axis

e. In producting compound lenses the several

types of glass and the curves of the various faces are

may be any of a wide variety of patterns, an egg-1, pear-,

carefully chosen to give approximate correction for coma

coma-, or comet-shaped blur (fig 2-66). Its general

as well as for other aberrations. Aplantics are lenses

appearance of a comet gives it the name of coma.

corrected for spherical, coma and chromatic aberration.

When viewed under a microscope, a point of light

influenced by coma may have a very fantastic shape due

2-42. Curvature of Image and Distortion.

to its being affected at one time by all types of

a. Definition of Distortion. Distortion is a form of

aberrations. Inasmuch as coma causes portions of

spherical aberration in which the relative location of the

points of light to overlap others, the result is blurred

images of different points of the object is incorrect.

images of objects in the portion of the field affected by

When imaged by a lens having this defect, a straight line

coma.

extending across the field is curved. This is a serious

defect in instruments of high magnifying power because

the amount of distortion present is increased in

proportion to the power of the instrument.

b. Causes of Distortion. Distortion (fig. 2-67) is

particularly harmful when the object is held in close

proximity to lens and is caused by rays of light from

different points of the object being refracted by dissimilar

portions of a lens. When a line extends across the field,

is located close to lens, and is off clear, rays from the

middle of the line strike the lens nearer its center. These

rays are refracted at a different angle from that of rays

striking near the margin of the lens, giving a curved

AR910072

instead of straight appearance to the image.

Figure 2-66. Appearance of coma greatly magnified

2-49