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A failing in the ability of a lens to produce a true image.  There are many forms of aberration and the lens designer can often correct some only by allowing others to remain.  Generally, the more expensive the lens, the fewer aberrations it has.  While no single lens is called a 'perfect lens', the "ideal" lens would reproduce a subject in a faithful, clearly defined image.

Aberrations, which can be divided into six basic faults, affect the ideal performance in an optical system.


Metering feature that is used to hold the exposure setting when used in the automatic mode.  Used most commonly in a situation where off centring of the subject in composition and wish to retain the exposure setting of the subject OR where the level of exposure reading both the subject of interest and the background exposure reading is different eg. back lighting.  Used to hold an automatically controlled shutter speed and/or aperture. Recommended when the photographer wants to control an exposure based on a scene's particular brightness area with centre weighted or spot metering.

AE-L (Automatic Exposure Lock)

The area of a scene that a lens covers or sees.  Angle of view is determined by the focal length of the lens.  A wide-angle lens (short focal length) includes more of the scene.  A telephoto lens (longer focal length) has a narrower angle of view.  See our tutorial on angle of view.

Angle of view

Having the ability to bring all colours of the visible spectrum to a common plane of focus, within close tolerances, usually refer to a lens with such superior colour correction. Also refer to "ED", "LD", "SD", "UD".

APO (Apochromatic)

The hole or opening formed by the metal leaf diaphragm inside the lens or the opening in a camera lens through which light passes. The size of aperture is either fixed or adjustable. Aperture size is usually calibrated in f numbers.  The larger the number, the smaller the lens opening.  Sometimes referred to as the diaphragm or the iris.

Aperture affects depth of field, the smaller the aperture, the greater is the zone of sharpness.  See our tutorial on f numbers and stops.


An exposure mode on an automatic or autofocus camera that lets you set the aperture while the camera sets the shutter speed for proper exposure.  If you change the aperture, or the light level changes, the shutter speed changes automatically.  Apart from the sport or action arena, aperture priority is probably the most common automatic preference in photography.

Aperture priority