GN (Guide number)
Used to express the power output of the flash nit. It indicates the power of a flash in relation to ISO film speed. Guide numbers are quoted in either meters or feet. Guide numbers are used to calculate the f/stop for correct exposure as follows: Number calculated by multiplying proper flash exposure aperture by the subject distance.
Hyperfocal Distance
Distance of the nearest object in a scene that is acceptably sharp when the lens is focused on infinity.
Infinity.
Infinite distance. In practice, a distance so great that any object at that distance will be reproduced sharply if the lens is set at its infinity position, i.e. one focal length from the digital sensor.
Interchangeable lens.
Lens designed to be readily attached to and detached from a camera.
ISO Speed
The international standard for representing digital sensor sensitivity. The emulsion speed (sensitivity) of the digital sensor as determined by the standards of the International Standards Organization. In these standards, both arithmetic (ASA) and logarithmic (DIN) speed values are expressed in a single ISO term. For example, a digital sensor with a speed of ISO 100/21° would have a speed of ASA 100 or 21 DIN. The higher the number, the greater the sensitivity, and vice versa. A digital sensor speed of ISO 200 is twice as sensitive as ISO 100, and half that of ISO 400 digital sensor.
Lens Speed
The largest lens opening (smallest f-number) at which a lens can be set. A fast lens transmits more light and has a larger opening than a slow lens. Determined by the maximum aperture of the lens in relation to its focal length; the "speed" of a lens is relative: a 400 mm lens with a maximum aperture of f/3.5 is considered extremely fast, while a 28mm f/3.5 lens is thought to be relatively slow.
Macro Lens
A lens that provides continuous focusing from infinity to extreme close-ups, often to a reproduction ratio of 1:2 (half life-size) or 1:1 (life-size). Nikon's version for their "macro" is "micro", eg. 105mm F2.8 Micro-Nikkor.
