Glossary of Photographic Terms
A
AI
Automatic maximum-aperture indexing. A system developed by Nikon that enables the aperture setting on an F-mount lens to be communicated mechanically to the camera body by way of a rotating tab in the mount without the need for the lens to be “indexed” to the camera when initially mounted. AI debuted with the Nikkormat FT3 in 1977.
Aperture
Aperture refers to:
The opening within a photographic lens that allows light to travel through it. The aperture controls two fundamental things about this light: (i) how much of it enters (per unit time), and (ii) how parallel (or, “collimated”) the rays are. The larger the aperture, the more light enters and the less collimated (more cone-like) it is; the smaller the aperture, the less light enters and the more collimated (straight line-like) it is. The quantity of light entering is a primary determinant of exposure, and the degree of collimation affects depth-of-field.
The ratio between the area of the opening and the focal length of the lens, also known as f-number, or f-stop.
Aperture-Priority, or Aperture-Preferred
An auto-exposure mode in which the camera automatically set the appropriate shutter speed for the selected aperture and ISO. Aperture-priority auto-exposure is typically the preferred auto-exposure mode when the photographer is primarily concerned about depth-of-field. The intention may be to widen the depth-of-field, e.g., in a landscape photograph (in which case a high f-number will encourage the camera to calculate a slow shutter speed for correct exposure), or to narrow the depth of field, e.g., in a portrait photograph (in which case a small f-number will encourage the camera to calculate a high shutter speed.)
Aperture Ring
The collar on a photographic lens that permits the photographer to control the size of the aperture by opening or closing the iris. This collar is typically marked with f-numbers (1.4, 2, 2.8, 5.6, etc.) and may feature detents (“clicks”) that prevent inadvertent movement of the collar from the set position; these give positive haptic and aural feedback.
ASA
American Standards Association.
Auto-Exposure
Any shooting mode in which the camera automatically sets exposure parameter(s) on behalf of the photographer. Auto-exposure modes include aperture-priority automatic, shutter-priority automatic, and program automatic.
D
Depth-of-Field
E
Exposure
F
f-Number, or F-Stop
Film Speed
Focal Length
Focus
I
Iris
ISO
International Standards Organization.
P
Program Automatic
An auto-exposure mode in which the camera automatically sets both aperture and shutter speed for the selected ISO.
S
Shutter-Priority, or Shutter-Preferred
An auto-exposure mode in which the camera automatically sets the aperture given the selected shutter speed and ISO. Shutter-priority auto-exposure is typically the preferred auto-exposure mode when the photographer is primarily concerned about motion in the field. The intention may be to freeze motion (in which case a high set shutter speed will encourage the camera to calculate a large aperture for correct exposure), or to blur motion (in which case a low set shutter speed will encourage the camera to calculate a small aperture.)