Chromatic aberration

Colour fringing is often seen on high contrast edges in photographs. Caused because light rays of different wavelengths focus at different distances from the lens (blue will focus at the shortest distance and red the furthest) stopping down the lens doesn’t improve it. However, it is correctable through the use of low dispersion (ED, LD, SD) glass.

Colour temperature

A description of the colour of a light source generated by comparing it with the colour of light emitted by a (theoretical) perfect radiator at a particular temperature. "Photographic daylight" has a colour temperature of about 5500K. Photographic tungsten lights have colour temperatures of either 3400K or 3200K depending on their construction.

Depth of Field

The zone of acceptable sharpness in front of and behind the subject on which the lens is focused. It is dependent on aperture, focal length and focused distance. The wider the aperture, the longer the focal length and the closer the focused distance, the shallower the depth of field.

Distortion

A lens aberration or fault which does not affect the sharpness of the image but alters the shape of objects. There are two types of distortion: barrel (where straight lines are bowed in at the edges of the picture frame resembling the sides of a barrel) and pincushion (the opposite of barrel distortion where straight lines are bowed in toward the middle to resemble the sides of a pincushion).

ED (Extra Low Dispersion)

Glass with ED properties that limit or correct light rays passing through the lens to minimise chromatic aberration.

EV (Exposure Value)

A method of quantifying scene brightness. EV1 is defined as a 1 second exposure at f/1.4 and ISO 100.

Exposure

The quantity of light allowed to act on a photographic material. A product of the intensity (controlled by the lens opening), the duration (controlled by the shutter speed or enlarging time) of light striking the photographic material and the sensitivity (ISO) of that material.

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