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The inability of a lens to produce a perfect, sharp image, especially towards the edge of the lens field. This is because each colour in the optical spectrum cannot be focused at a single common point on the optical axis. These faults can be reduced by compound lens constructions, and the use of small apertures.

Also known as Chromatic Aberration, Purple Fringing

Circular mount, available in several sizes, enabling accessories such as filters to be used with lenses of different diameters.

Flaws in the image generated by the camera, either by the sensor and camera processing or by the storage method. JPEG is well known for artefacts as it compresses the image. Artefacts can also take the form of "jaggies" on diagonal lines, which make them appear to staircase.

The original system of rating photographic materials, which was devised by the American standards Association. The ISO rating system is now used in place of the ASA.

This is where straight lines appear to bow outwards when near the edge of the frame during wide angle shots.

Distance between the nearest point and the farthest point in the subject which is perceived as acceptable sharp along a common image plane.

Distance which the film plane can be moved while maintaining an acceptably sharp image without refocusing the lens.

Numbers on the lens barrel indicating the size of the aperture relative to the focal length of the lens. f numbers are calculated by dividing the focal length of the lens by the effective diameter of the aperture.

In this example the lens is a 70-300mm focal length and its f numbers are 1:4-5.6, which means that at 70mm the widest aperture is f/4 and at 300mm the widest aperture is f/5.6. 

Wider apertures are generally more expensive to produce.

Distance between the rear focal point of the lens and the focal plane, when the focus is at infinity. This is also known as telephoto or zoom, and can be given in milimeters or as a "times six" reference. x6 refers to 35mm film lens as a base (i.e 1x = 35mm, 2x = 70mm etc…)

Imaginary line perpendicular to the optical axis which passes through the focal point. It forms the plane of sharp focus when the lens is set at infinity.

This is a number that represents the power of a flashgun. You can use it to work out what lens aperture to use for a given subject distance by dividing the guide number by the distance in metres. These days camera can calculate this automatically.

ISO (International Organization for Standardization) is the measure of a photographic film’s sensitivity to light. Film with lower sensitivity (lower ISO speed rating) requires a longer exposure and is thus called a slow film, while film with higher sensitivity (higher ISO speed rating) can shoot the same scene with a shorter exposure and is called a fast film.

Film speed is roughly related to granularity, the size of the grains of silver halide in the emulsion, since larger grains give film a greater sensitivity to light. Fine-grain stock, such as portrait film or those used for the intermediate stages of copying original camera negatives, is “slow”, meaning that the amount of light used to expose it must be high or the shutter must be open longer. Fast films, used for shooting in poor light or for shooting fast motion, produce a grainy image. The image actually consists of a mosaic of developed and undeveloped areas of the emulsion, and each grain of silver halide develops in an all-or-nothing way.Thus, film is a threshold detector rather than a linear detector. If the subject has an edge between light and darkness and that edge falls on a grain, the result will be an area that is all light or all shadow. An accumulation of such areas breaks up the visible contours of the object, the effect known as graininess (or grain).

This is the opposite of Barrel distortion. Straight lines appear to bow inwards when near the edge of the frame. You often see this with zoom lenses at or near there maximum focal length.