a) An adjustable opening in a lens which, like the iris in the human eye, controls the amount of light entering a camera. The size of the aperture is controlled by the iris adjustment and is measured in f-stops. A smaller f-stop number corresponds to a larger opening that passes more light.

b) As applied to ATV, the finite size and shape of the point of the electron beam in a camera or picture tube. As the beam does not come to an infinitesimal point, it affects the area around it, reducing resolution.