The adjustable opening —or f-stop— of a lens, which determines how much light passes through the lens on its way to the film plane, or nowadays, to the surface of the camera's imaging sensor.

Aperture is referred to the lens diaphragm opening inside a photographic lens. The size of the diaphragm opening in a camera lens regulates the amount of light passes through onto the film inside the camera the moment when the shutter curtain in camera opens during an exposure process. The size of an aperture in a lens can either be a fixed or the most popular form in an adjustable type (like an SLR camera). Aperture size is usually calibrated in f-numbers or f-stops. i.e. those little numbers engraved on the lens barrel like f22 (f/22),16 (f/16), f/11, f/8.0, f/5.6, f/4.0, f/2.8, f/2.0, f/1.8 etc. Each of this value represents one time the amount of light either more or less in quantity. Meaning to say, f/16 will let in 1X the amount of light than a diaphragm opening of f/22 and so forth; while on the other hand, an aperture of f/4.0 will let in 1X lesser than that of f/2.8 etc.

Setting the aperture on the camera will also effect the depth of focus in pictures. When using a telephoto lens, a small aperture number will make the background get blurry, so the foreground subject is more defined. This is commonly used in portraiture. Using a wide angle lens with a large aperture number will make the subject matter be in focus form front to back. This is a great option for photographing scenery and landscapes.