| ASA | American Standards Association |
| Artefacts | Spurious effect or object appearing in an image, due to defects in the collection, processing, or display of an image |
| art history brush | The art history brush tool lets you paint with stylized strokes, using the source data from a specified history state or snapshot. |
| Apochromat | A lens corrected for chromatic aberration (color fringing) at three wavelengths in the visible spectrum. |
| Aperture Priority | (A) Exposure mode where the camera determines the optimum aperture for you. |
| Aperture | The variable diameter hole used to control the amount of light passing through a lens. |
| APD | Adobe Photo Downloader |
| anti-alias | the technique of minimizing the distortion artifacts known as aliasing when representing a high-resolution signal at a lower resolution |
| Anodisation | An electrolytic process for forming a stable film of aluminum (aluminum) oxide on the surface of aluminum alloys. Many underwater components are anodised to protect them from seawater. |
| annotations | You can attach note annotations (notes) and audio annotations to an image in Photoshop. |
| angle of view | (AOV) The angle that a lens can 'see' usually measured across the diagonal of the frame |
| Angle Of Flash Coverage | The area that a flash will illuminate is known as the angle of coverage |
| Angle Of Coverage | The angle between rays of light entering a lens from the extreme corners of a scene. AKA angle of view (AOV) |
| Analog video | Video signals based on fluctuating voltages |
| Amp Hours | Ah, Ampere hours The capacity rating of an electrical cell or battery. |
| Amp | Ampere, A The unit of electrical current, i.e. quantity of electricity per unit time. Current flows through wires and conductors, whereas voltage appears across things like generators, batteries and resistors. |
| Ambient Light | natural light from the Sun |
| Alpha channel | Additional color channel used for storing masks and selections |
| Alkaline | standard, non-rechargeable batteries |
| Algorithm | A mathematical procedure. A sequence of mathematical operations. |
| Airbrush | The Airbrush option in the Brushes palette lets you apply gradual tones to an image, simulating traditional airbrush techniques. |
| AF lock | Autofocus lock. You can prefocus at a distance, which is then kept regardless of subject movement. |
| AF | Automatic Focus. |
| AE lock | Auto Exposure lock |
| AE | Automatic Exposure. A method of linking the camera exposure settings to the ambient or reflected light level. Automatic exposure relies on assumptions about the reflectivity of the subject, and it cannot anticipate the photographer's artistic intentions! |
| Adobe Photo Downloader | Adobe’s download utility used for transferring photos from card or camera to computer. The utiity ships with Adobe Bridge |
| Adobe gamma | A calibration and profiling utility supplied with Photoshop |
| Adobe Camera Raw | a program in Adobe creative Suite for editing RAW files |
| Adjustment Layer | an adjustment to the image saved as a special type of layer |
| Additive Colors | Red, Green, and Blue |
| ADC | Analog to Digital Converter (ADC). |
| Actions | An action is a series of commands that you play back on a single file or a batch of files. For example, you can create an action that applies an Image Size command, followed by an Unsharp Mask filter, and a Save command that saves the file in the desired format. |
| Acrylic | Poly methyl methacrylate. A clear plastic with good optical properties. Used for many dome ports. Trade names Perspex, Plexiglass. |
| ACR | Adobe Camera Raw (a program in Adobe creative Suite for editing RAW files) |
| Achromatic Doublet | A lens in which achromatic correction is achieved by cementing together two lenses made from different types of glass. |
| Achromat | A lens corrected for chromatic aberration (color fringing) at two wavelengths in the visible spectrum. Most modern camera lenses are achromats. |
| AC | Alternating Current. An electrical current which changes over time, usually in a sinusoidal fashion. Usually associated with mains electricity. |
| ABS | 'acrylonitrilebutadienestyrene copolymer' a cross between plastic and rubber with excellent engineering properties, used to make underwater housings, cabinets for electronic equipment, solventweld plumbing fittings, etc. |
| Aberration | Usually used in the context on lens performance to describe an imperfection. Deviation, Wandering away, The nonconvergence of rays of light to one focus. |
| AA Filter | Most digital SLR cameras employ a Low Pass Filter (LPF) or Anti-Aliasing (AA) filter in front of the imager to help eliminate color aliasing (moire) problems. |
| A/D Converter | A device that converts analog information (a photograph or video frame) into a series of numbers that a computer can store and manipulate. All digicams use an A/D converter, the higher the bit rate the better the output. Modern hi-res digicams employ a 12-bit or 14-bit A/D to increase the dynamic range (range of light from highlight to shadow) |
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