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.

Comma
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.

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)

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.