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Glossary and TLAs

 

AE

See Auto-exposure.

AF 

See Auto-focus.

Alpha channel

Color images require three color channels. For graphic applications, a so-called alpha channel may be added to contain a transparency value. The alpha channel functions as a mask in layered images to control the amount by which each pixel value is combined with those of corresponding pixels in other layers.

Ambient light

The existing illumination of a scene, as distinct from illumination (such as studio lighting) added by the photographer.

Aperture

The diameter of the hole in the diaphragm of a photographic lens, measured in proportion to its focal length. For example, f/4 indicates one-quarter of the focal length, so that the diameter of the hole would be 25mm in the case of a lens with a focal length of 100mm. The absolute diameter, however, is little use to photographers. The useful number is the relative diameter: 4 in this case. Check also the articles on Exposure and Advanced Exposure.

Auto-exposure

A feature of some modern cameras that sets exposure automatically - see also Av mode, Program mode and Tv mode.

Auto-focus

A mechanism by which a camera may be able to focus automatically. There are numerous different systems. Active systems employ IR light or ultrasound, bounced back from the target, to measure the distance to the subject. Some of these can be fooled by intervening glass windows. Passive systems identify points of high contrast in the subject and adjust the lens to minimize blurring. Early passive systems were slower than competing active systems. Combined systems have also been used.

Av mode

Aperture priority mode, often provided as a feature on AE cameras. The photographer sets the aperture and the camera determines the appropriate shutter speed with its built-in light-meter. See also Exposure compensation.

Available light

The existing illumination of a scene: photography by available light is photography without use of artificial illumination.

B

Marking on shutter speed dials to indicate bulb exposure (q.v.).

Backlight

Lighting from behind the subject, i.e. from the opposite side from the camera.

Bracketing

The practice of taking several photos at slightly different camera settings to ensure an acceptable result: most usually applied to exposure, bracketing can also be applied to focus and/or white balance (q.v.).

Bulb exposure

Long exposure setting: the shutter remains open so long as pressure is maintained on the shutter release. In early cameras, the shutter was operated by squeezing a rubber bulb on the end of a tube.

CCD

Charge-Coupled Device: a technology used in manufacturing digital sensors.

Channel

See Color channel.

Circle of Confusion

Please refer to the article Depth of field for details.

CMOS

Complementary Metal Oxide Semiconductor: a technology used in manufacturing digital sensors.

CMYK

Cyan - Magenta - Yellow - blacK: The standard color channels for 'four-color' commercial printing.

COC 

Circle of Confusion - please refer to the article Depth of field for details.

Color channel

In digital photography, color images have three color channels, usually red, blue and green (RGB). The combination of the RGB values for each pixel determines the resulting color. See also Alpha channel.

Continuous shooting

A camera setting that causes shots to be fired continuously as long as the shutter button is depressed. The frame rate typically ranges from about 3 to over 8 frames per second (fps). Digital cameras may fill their memory buffer after a certain number of shots and then slow down dramatically. Film cameras using a motor drive tend to run out of film very rapidly unless provided with large capacity bulk film systems.

Cropping

Selection of part of an image and removal of the unselected portion: cropping may be performed by physically trimming a print, by enlarging just part of a negative, by masking a transparency (slide), or digitally.

Depth of field

Please refer to the article Depth of field for details.

Desaturation

Reduction (total or partial) of saturation (q.v.). Total desaturation results in a monochrome (black and white, or grayscale) image.

Diaphragm

The mechanism which regulates the aperture (q.v.) of a photographic lens

Diffraction

Diffusion

Scattering and softening of light by means of translucent material (such as cloth) placed between a light source and the subject.

Diopter

Unit of measure of the power of a lens; equals the reciprocal of the focal length measured in metres. For example, the power of a lens with a focal length of 100mm (1/10 of a meter) is 10 diopters.

DOF

Depth of field

DSLR

Digital Single-Lens Reflex (camera) - please refer to the article Camera types for further information.

Exposure

The quantity of light allowed to reach the film or sensor. Please refer to the articles on Exposure and Advanced Exposure for further information.

Exposure compensation

Adjustment of exposure relative to the default meter reading of cameras in AE mode, usually in a range of +/- 2 to +/- 3 stops. In full auto or program mode, the camera's internal logic decides whether aperture or shutter speed is modified. In Av mode, the shutter speed is modified. In Tv mode, the aperture is modified.

Field of view

The angle of view of a scene included in the image by a lens. FOV may be measured horizontally (landscape format) or on the diagonal. Please refer to the article Focal Length and Field of View for details.

Fisheye lens

An extreme wideangle lens (q.v.), with FOV (q.v.) of 180°, at least on the diagonal, that exhibits spherical perspective, an effect similar to extreme barrel distortion.

Flash

A form of lighting that emits a short burst of light. Modern flash systems are invariably electronic. May be built-in to the camera, mounted on the camera as an accessory, or supported independently in the case of large and powerful units, such as are often used in studio work.

Focal length

The distance from the optical centre of a lens to the image plane when the subject is far away "at infinity". The normal focal length for any camera format is roughly equal to the length of the image diagonal. Please refer to the article Focal Length and Field of View for details.

FOV

See Field of view.

Frame rate

See Continuous shooting.

Grayscale 

See Desaturation.

Histogram

In general, a frequency chart; in photography, a frequency chart of the number of pixels in a digital image at each level of brightness, from black to white - histograms may also show the brightness ranges of each individual color channel. Many digital cameras can show a histogram of the image just taken, or (less commonly) about to be taken.

Hue

Technical word for color

Hyperfocal distance

The nearest point of exact focus for which a distant subject "at infinity" is still in acceptable focus. If focus is set at the hyperfocal distance, the nearest point in acceptable focus will be half the hyperfocal distance away. This setting provides maximum DOF. Please refer to the article Depth of field for details.

IR

Infra-Red

LCD

Liquid Crystal Display

Light table

A translucent surface, illuminated from below, used to examine slides.

Macro 

Please refer to the article Macro Photography for details.

Macro lens

A lens having the close-focusing ability required for Macro Photography.

Megapixel

One million pixels: a measure of the resolution of a digital camera's sensor. Sensor resolution determines the maximum image size of acceptable quality the sensor can deliver. Roughly speaking, with no special digital trickery, a quality image up to about 15sq.in. (100sq.cm.) is possible per one megapixel, without cropping.

MF Manual Focus. See also AF, Auto-focus.
Monochromatic

Single colored.

Monopod

A camera support with a single leg. A monopod takes the weight of the camera and lens, and provides a significant degree of steadiness, but must be held by the photographer.

Motor drive

The (usually optional accessory) mechanism for a film camera that allows Continuous shooting.

MP

See Megapixel.

Negative

Most black-and-white film (and many popular color films) use a negative-positive process. The film image is a negative, with its tones inverted. In negative color film, the colors are complementary, and overlaid with an orange-colored mask. A second inversion at print time restores normal tonality and color.

Normal lens

See Standard lens.

OOF

Out of focus

Over-exposure

Exposure (q.v.) in excess of the desired amount: the negative is too dense and/or the image is too light.

P&S

Point-and-Shoot (camera)

Parfocal A zoom lens (q.v.) that retains focus throughout its focal length range is called a parfocal lens.
Perspective 

(1)The impression of depth in a two-dimensional image; (2) The form of mapping from subject space to image space - see rectilinear perspective, spherical perspective.

Pixel

An abbreviation for Picture Element - each individual dot in a digital sensor or image is a pixel.

Pixel value

See Color channel

Power (of lens)

See Diopter

Print film

Negative (q.v.) color film. Most popular print films use C-41 processing, or equivalent.

Program mode

An automatic mode, often provided as a feature on AE cameras, which still allows the photographer to control a limited range of settings, as distinguished from fully automatic operation, in which the user retains no control over exposure at all. The camera determines both aperture and shutter speed with its built-in light-meter. See also Exposure compensation.

Rectilinear perspective

The perspective (q.v. 2) of the image from a normal, rectilinear lens: same as the perspective of a pinhole camera. In the absence of aberrations, straight lines are mapped into straight lines.

Resolution

Please refer to the article Resolution for information.

RF

Rangefinder (camera) - please refer to the article Camera types for further information.

RGB 

Red - Green - Blue: the usual color channels of digital photography; see also CMYK.

Saturation

The intensity of a color. Less saturated colors look muted. Pastel shades are pale, unsaturated colors.

Sensel SENSor ELement, i.e. an individual photosite on the sensor of a digital camera.
Sensor The electronic component in a digital camera which makes a digital record of the image formed by the lens, fulfilling the role of film in a traditional camera.
Shutter speed

A measure of the time during which the shutter is open during exposure. Short exposures of less than a second are measured in fractions of a second. Modern cameras generally provide for fast shutter speeds at least down to 1/1000" and often to 1/4000" or less. Longer exposures are measured in seconds, or (rarely) minutes or hours.

Slide

Mounted transparency.

Slide film

Film designed to produce a positive image, usually in color. Most popular slide films use E-6 processing, or equivalent.

SLR

Single-Lens Reflex (camera) - please refer to the article Camera types for further information.

Spherical perspective

The (approximate) perspective of a fisheye lens (q.v.), similar to the reflection seen in a polished sphere. Straight lines not passing through the lens axis are mapped as curves in the image.

Standard lens

A lens of normal focal length (q.v.)

T Marking on (some) shutter speed dials to indicate time exposure (q.v.).
Telephoto lens

A lens designed so that its physical length is less than its focal length (q.v.). Less strictly, any lens of longer-than-normal focal length, and less-than-normal FOV (q.v.).

Time exposure 1. Long exposure setting: the shutter opens when released and remains open until activated a second time. 2. In general, any long exposure may be described as a 'time exposure'.
TLA

Three-letter acronym - not all of which necessarily have exactly three letters J

TLR

Twin-Lens Reflex (camera) - please refer to the article Camera types for further information.

Transparency

(1) Quality of material (such as glass) that allows light to pass through; (2) Degree of same; (3) Positive film image, often mounted as a slide for purpose of projection.

Tripod

A camera support with three legs.

Tv mode

Shutter (time value) priority mode, often provided as a feature on AE cameras. The photographer sets the shutter speed and the camera determines the appropriate aperture with its built-in light-meter. See also Exposure compensation.

Under-exposure

Exposure (q.v.) of less than the desired amount: the negative is too thin and/or the image is too dark.

UV

Ultra-Violet

White balance

A control available on many digital cameras to adjust image color according to prevailing lighting conditions, so as to optimize fidelity to the original subject colors.

Wideangle lens

A lens of less-than-normal focal length (q.v.), and greater-than-normal FOV (q.v.).

Zoom lens

A lens with continuously adjustable focal length (q.v.) over a stated range. See also Parfocal.

Zoom ratio

The ratio of maximum to minimum focal length (q.v.) of a zoom lens (q.v.)