Different apertures of a lens
Diagram of rays at a surface, where is the angle of refraction.
Definitions of Aperture in the 1707 Glossographia Anglicana Nova
Simple ray diagram showing typical chief and marginal rays
Alvin Clark polishes the big Yerkes Observatory Great Refractor objective lens, with 40 inches 102 cm across, in 1896.
Rays and wavefronts
Diagram of decreasing aperture sizes (increasing f-numbers) for "full stop" increments (factor of two aperture area per stop)
The aperture range of a 50mm Minolta lens, f/1.4–f/16
Aperture mechanism of Canon 50mm f/1.8 II lens, with five blades
{{f/|32}} – small aperture and slow shutter
{{f/|5.6}} – large aperture and fast shutter
{{f/|22}} – small aperture and slower shutter (Exposure time: 1/80)
{{f/|3.5}} – large aperture and faster shutter (Exposure time: 1/2500)
Changing a camera's aperture value in half-stops, beginning with {{f/|256}} and ending with {{f/|1}}
Changing a camera's aperture diameter from zero to infinity

More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.

- Aperture

The marginal ray (sometimes known as an a ray or a marginal axial ray) in an optical system is the meridional ray that starts at the point where the object crosses the optical axis, and touches the edge of the aperture stop of the system. This ray is useful, because it crosses the optical axis again at the locations where an image will be formed. The distance of the marginal ray from the optical axis at the locations of the entrance pupil and exit pupil defines the sizes of each pupil (since the pupils are images of the aperture stop).

- Ray (optics)
Different apertures of a lens

1 related topic

Alpha

The image side of the lens of an SLR camera; the exit pupil is the light area in the middle of the lens.

Exit pupil

The image side of the lens of an SLR camera; the exit pupil is the light area in the middle of the lens.
The aperture of this system is the edge of the objective lens. The exit pupil is an image of it.
The small exit pupil of a 25×30 telescope and large exit pupils of 9×63 binoculars suitable for use in low light
The exit pupil appears as a white disc on the eyepiece lens of these 8×30 binoculars. Its diameter is 30 ÷ 8 = 3.75 mm.

In optics, the exit pupil is a virtual aperture in an optical system.

Only rays which pass through this virtual aperture can exit the system.