aperture

In photography, an opening in the camera that allows light to pass through the lens to strike the film. Controlled by the iris diaphragm, it can be set mechanically or electronically at various diameters.

The aperture ratio or relative aperture, more commonly known as the f-number, is a number defined as the focal length of the lens divided by the effective diameter of the aperture. A smaller f-number implies a larger diameter lens and therefore more light available for high-speed photography, or for work in poorly illuminated areas. However, small f-numbers involve small depths of focus.