A special thanks to Doug Dever and Amy K for their invaluable help on this glossary.
Aberrations Causes completely blurred images. There are six types of aberrations: spherical, coma, astigmatism, curvature of field, distortion and chromatic.
AF (Autofocus) Lock Used to prevent autofocus operation once the subject is in focus.
AF Sensor The sensor used to detect focus in cameras equipped with autofocus operation.
APS Advanced Photo System. A photographic system that uses 24mm film with a magnetic layer to record exposure and other information. It can be printed in three formats, printing includes an "index print", and the film stays in the cassette after processing. Most APS cameras are point-and-shoots.
Aperture The adjustable opening in a camera lens used to control the amount of light reaching the film. The size of this hole is called the f-stop.
Apochromatic Refers to lenses designed to correct for color aberrations. Usually used in telephoto lenses that have large maximum apertures. (See Chromatic Aberration.)
ASA Abbreviation for American Standards Association. In conjunction with a number, e.g., ASA 400, refers to film "speed" or sensitivity. The higher the number, the more sensitive the film.
Aspherical Not a continuous curve. Refers to elements in lenses which have been designed to compensate for distortion by having different curves on individual elements. Allows for a more compact lens.
Astigmatism An optical defect that causes light rays from an off-axis point to form images at different positions.