Aberration Defect in a lens resulting in less than optimum sharpness over part of the image plane. See also Chromatic aberration, Spherical aberration.

Accelerator (also known as "activator" or "alkali" component) Chemical ingredient of developer to speed up the otherwise slow activity of developing agents. Normally an alkali such as sodium carbonate, borax, or (high contrast developers) sodium hydroxide.

Accessory shoe Fitting on a camera to allow devices such as flashguns to be clipped on. See also Hotshoe.

Achromatic Lens system corrected to eliminate the effects of chromatic aberration.

Acid Chemical substances with pH below 7. Harmful to skin and eyes. Because acid neutralizes an alkali, acidic solutions are often used to halt ...

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