## Aperture

key: ** **` aprtr `
type: f

- f: the
*f*-number

## Description

The *f*-number is the ratio of the focal length to the diameter of the
aperture. When photographers write ``*f*/5.6'' that means the diameter
of the aperture is the focal length over 5.6; 5.6 is the *f*-number.
In real cameras with complicated lenses, the ``effective aperture'' is taken
to be the region on the front of the lens through which light rays can get to
the film. This is also called the ``entrance pupil.'' This is the aperture
that the `aprtr` chunk is meant to specify.

If the
aperture is not circular, then its area should be equal to the area of a
circle of this diameter.

## Restrictions

This chunk only makes sense inside a camera chunk.

## Example

See the camera example .