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Aberration

Optical defect in a lens (sometimes unavoidable) causing distortion or loss of sharpness in the final image.


Acid

A substance with a pH below 7. Since an acid neutralizes an alkali, a stop bath is usually an acidic solution that stops that action of the alkaline developer.


Adapter ring

A ring used to attach one camera item to another; for example, to attach a lens to a camera in reverse position in order to increase image sharpness when focusing very close to the subject.


Agitate

To move a solution over the surface of film or paper during the development process so that fresh liquid comes into contact with the surface.


Angle of View

The amount of a scene that can be recorded by a particular lens from a given position; determined by the focal length of the lens.


Aperture

The size of the lens opening through which light passes.


Aperture control

The ring on the camera lens (a push button on some models) that, when turned, adjusts the size of the opening in the iris diaphragm and changes the amount of light that reaches the film.


Aperture-priority mode

An automatic exposure system in which the photographer sets the aperture (f-stop) and the camera selects the shutter speed for normal exposure.


ASA

A film speed rating similar to an ISO rating


Automatic exposure

A mode of camera operation in which the camera automatically adjusts either the aperture, the shutter speed, or both for normal exposure.


Automatic flash

An electronic flash unit with a light-sensitive cell that determines the duration of the flash for normal exposure by measuring the light reflected back from the subject.


Available light

A general term implying relatively dim light that already exists where a photograph is to be made.


Averaging meter

An exposure meter with a wide angle of view. The indicated exposure is based on an average of all the light values in the scene.