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aberration—The failure of a lens to bring all the rays of light to exact focus, causing a blue-red image.
achromatic—A lens which corrects for chromatic aberration; transmits light that forms images practically free from prismatic colors.
aplanatic—A lens which corrects for spherical aberration and coma.
astigmatism—A defect in which the lens fails to unite rays of light from an external point at a single image point, thus giving an imperfect image or vision.
chromatic aberration—The inability of a lens to focus light of different colors at a simple point.
Coddington—A corrected lens, named after its British inventor, Henry Coddington.
coma—The blurred appearance or hazy border surrounding an object viewed through a lens which is not free from spherical aberration.
concave—Describes a lens surface which is hollowed; interior of a curved surface.
convex—Describes a lens surface that curves or is rounded outward.
corrected—A lens or lens system which corrects for aberrations; remedies deviations of light rays from object to eye to produce a clear, sharp image.
crown glass—Optical alkali-lime glass having a low dispersion and usually a low index of refraction.
curvature of field—When a plane field is not imaged as a plane, or the outer part of the field is not imaged in the same plane as the center and therefore appears out of focus; as opposed to flatness of field.
curvature of lens—The amount of sharpness of curve in a lens surface.
diopter—The amount of power in a lens needed to focus parallel light at one meter.
dispersion—The separation of light into its component colors, as in passing through a prism.
distortion—That defect of a lens whereby the images of straight lines appear curved.
double lens magnifier—A magnifier composed of two single lenses.
flatness of field—Appearance of the image to be flat; a plane in the object will be imaged as a plane as opposed to curvature of field.
flint glass—A heavy, brilliant glass containing lead and having a high dispersion and usually a high index of refraction.
focus—The point at which light rays through a lens intersect to form an image.
Hastings Triplet—A highly corrected magnifier composed of three simple lens elements cemented together to form a single lens.
highly corrected—A magnifier or lens in which virtually all aberration is eliminated.
image—The likeness or picture formed by a lens; the optical counterpart of an object.
meniscus—A crescent-shaped lens—one which is concave on one surface, convex on the other. It may be converging or diverging.
plano—Pertaining to flat; a plano lens surface has no curve.
plano-concave—A lens with one surface flat, the other curved inward. (See concave)
plano-convex—A lens with one surface flat, the other curved outward. (See convex)
refractive index—The ration of speed of light in a vacuum, or in a given medium to its speed in a different medium.
semi-corrected—A magnifier or lens in which only part of the aberration is eliminated.
spherical aberration—A defect in a lens which causes marginal and central rays to focus at different distances from the lens, producing an image which lacks contrast.
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