Aperture is the opening that controls how much light reaches a camera sensor. It is written as an f-number, and a lower number means a larger opening. A wider aperture admits more light for night work but reduces depth of field, so less of the scene stays sharp from near to far.
Aperture is one of the three settings that decide exposure, along with shutter time and sensor sensitivity. In a fixed security camera it is chosen at manufacture and cannot be adjusted later.
F-numbers and depth of field
Each step in the f-number sequence halves or doubles the light. Small numbers like f/1.4 admit a great deal and leave a narrow band in focus; large numbers like f/11 admit less and keep more of the scene sharp. Security cameras usually favor light gathering over a shallow look.
Aperture in fixed and varifocal lenses
A fixed-lens camera has one aperture, so its low-light behavior is fixed too. A varifocal lens holds an aperture setting while the focal length is adjusted. Fast lenses help in dim scenes, though sensor sensitivity and noise reduction affect the result just as much.
Related:Commercial security camera systemsGlossary: PTZ camera systems