Digital Photography Glossary

Aberration
Essentially, anything in an image that is not supposed to be there. For example, optical imperfections within a lens may cause distortions  such as barrel distortion or pincushioning. Barrel distortion, the outward curvature or bowing of straight lines, is most noticeable in wide-angle lenses or at the extreme wide-angle end of zoom lenses. Pincushioning, the inward curvature or bowing of straight lines (almost as if the image were pinched), is most common in telephoto lenses or the extreme telephoto end of zoom lenses. When corrective elements are incorporated into a lens design, most spherical aberrations can be minimized. Additionally, these spherical aberrations can often be adjusted with image-editing software. (See also Chromatic Aberration.)

AD Converter
The Analog to Digital Converter (ADC) converts the light waves that are absorbed by each photodiode (pixel) on an image sensor into digital code. This digital information is then processed into image data that is then stored on the camera’s media card or internal memory. This image data can then be read as a photograph by a camera or computer.

Aperture
Measured in f-stop numbers, it is the variable opening in a lens that controls the amount of light that hits the image sensor, thereby affecting exposure. A small f-stop number (i.e., f/2.8) represents a wide lens opening, which allows more light to pass through the lens to the sensor and is beneficial in low-light conditions. A larger f-stop number (i.e., f/8.0) narrows the lens opening and allows less light to hit the sensor. The latter provides a broader depth of field, while the former decreases the depth of field.

APO (Apochromatic)
Lenses that use internal elements to bring all colors of the visible spectrum to a common point of focus, creating a sharp image and minimizing chromatic aberrations, are referred to as APO lenses. These lenses are generally more expensive than non-APO lenses.

Artifacts
Digital image  anomalies caused by the image sensor, optics or internal image processing of the camera. These  anomalies include: blooming (brightened highlights usually visible along high-contrast edges); maze or moiré artifacts (as the name implies, maze artifacts appear in a maze-like pattern, while moiré artifacts occur as wavy lines); chromatic aberrations (see below); jaggies (pixels visible in a stair-stepped pattern, usually along straight or curved lines); noise (see below); and sharpening halos (a bright or white area similar to blooming appears along an edge when an image has been oversharpened).

Autofocus (Active & Passive)
Active autofocus cameras bounce infrared beams off the subject to determine what the focal length of the lens should be. Passive autofocus cameras use the light falling on photocells to determine the correct focus. When a scene is out of focus, the intensity of light on each adjacent photocell is very similar. The microprocessor moves the lens until the contrast of light between each adjacent photocell is at its maximum.

Autofocus (Continuous)
Available on some digital cameras, this AF option signals the camera to constantly focus on whatever object is in front of the lens. While some cameras perform better than others, continuous AF is helpful when photographing a moving subject.