Focal length

The distance (in mm) between the film and the optical centre of the lens when the lens is focused on infinity. In 35mm format cameras, lenses with a focal length of approx. 50mm are called normal or standard lenses. Lenses with a focal length less than approx. 35mm are called wide angle lenses, and lenses with a focal length more than approx. 85mm are called telephoto lenses. Lenses which allow the user to continuously vary the focal length without changing focus are called zoom lenses.

Focus

The act of adjusting a lens to produce a sharp image. In a camera, this is performed by moving part of the lens towards or away from the film or by moving the front part of the lens towards or away from the rear part, thus altering its focal length.

Full aperture metering

TTL metering systems in which the camera simulates the effect of stopping down the lens, while leaving the diaphragm at full aperture to give full focusing screen brilliance.

GN (Guide number)

Used to express the power output of a flash unit. Guide numbers are quoted in either meters or feet. The guide number is used to calculate either the distance between the flash unit and the subject, or the appropriate f stop to use.

GN calc

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