The opening of a lens that admits light. Except in very simple cameras, the Aperture can be varied in size by an adjustable  diaphragm, which regulates the amount of light passing through the lens. The relative Aperture is measured as the focal length of the lens divided by the diameter of the Aperture; this is expressed as an f-number: f/3, f/5, and so on.

Aperature has a dramatic effect on the Depth-of-field of the photograph.