Aperture is the ratio of the size of the lens opening to the focal length of the lens.
Aperture is stated in ƒ numbers. Like shutter speed, each increment is twice or half the preceding. With aperture, unlike shutter speed (and because it is a ratio), smaller numbers mean a larger opening. A basic sequence of f/stops or apertures is ƒ/2, ƒ/2.8, ƒ/4, ƒ/5.6, ƒ/8, ƒ/11, ƒ/16, ƒ/22. The numbers double every other time, but each increment still represents a stop. The table below shows how the area of the lens opening, thus the amount of light able to enter the camera either doubles or halves (with some rounding) as the aperture changes.
|ƒ/#||Aperture diameter (mm)||Opening Area (mm²)|
Below is a scale illustration of aperture openings, from ƒ/2 on the left, through ƒ/22 on the right.
Aperture controls depth of field. Depth of field is the distance, both in front of and behind the plane of sharp focus, where objects remain in reasonable focus. In other words, how much of the image is in focus from near to far. In general, for a given lens, the smaller the aperture (the bigger the f/number), the more depth of field. Depth of field is also controlled by two other variables: the focal length of the lens, and the distance to subject. The shorter the focal length of a lens, the more depth of field at a given aperture and distance. The closer to the subject, the less depth of field with a given lens focal length and a given aperture.