"What Is Aperture? The main function of a camera lens is to collect light. The aperture of a lens is the diameter of the lens opening and is usually controlled by an iris.The larger the diameter of the aperture, the more light reaches the film / image sensor. Aperture is expressed as F-stop, e.g. F2.8 or f/2.8. The smaller the F-stop number (or f/value), the larger the lens opening (aperture). [Note: Many camera user manuals today will refer to the aperture in terms of "aperture value" instead of f/value. I'm not sure when this trend started but don't get confused between "aperture" and "aperture value." Aperture value" is simply another way of saying f/value.] In practice, unless you are dealing with a fixed-aperture lens (many simple point-and-shoot cameras have only one fixed aperture), the aperture of a lens is usually expressed as a range of fstops. When you read the specifications of a camera, the aperture may be expressed in a number of different ways, the following three being the most common: Maximum Aperture: Max. Aperture F2.8 This simply states that the maximum aperture for the lens is F2.8. Aperture Range: Aperture Range F2.8-F8.0 This states the max. and min. aperture, the assumption being that there are standard increments between them. Maximum Wide-Angle and Telephoto Apertures: Aperture F2.8(W)-F3.5(T) This gives the max. aperture for the wide-angle (F2.8) and telephoto (F3.5) focal lengths of a zoom lens. A "fast" lens is one that has a large maximum aperture (F2.4, F2.0 for current digital cameras; F1.4, F1.2 for 35mm film cameras)." source: http://www.photoxels.com/tutorial_aperture.html

There is no difference between the aperture in a film camera or in a digital camera. The aperture is the diameter of the adjustable iris that controls the amount of light passing through the lens and onto the film plane or digital sensor. An aperture is expressed as an f-stop, which is equal to Focal_Length / Aperture_Diameter, and is written: f/number. For example, if a lens has a focal length of 50mm - the 'normal' 35mm film camera lens - and the aperture diameter is 5mm, then the f-stop is 50/5 or f/10. If you know what f-stop a lens is set to, you can determine the physical diameter of the aperture. For example, with an aperture of f/11: - On a 50mm lens, the aperture is 4.55mm in diameter (50/11). - On a 100mm lens, the aperture is 9.09mm in diameter (100/11). - On a 20mm lens, the aperture is 1.82mm in diameter (20/11). F-stops are used because they are a standard way to express the amount of light passing through a lens. The amount of lens passing through a 50mm lens set to an aperture of f/11 is the same as the amount of light passing through a 300mm lens set to f/11. This is far simpler than measuring the diameter of the aperture to calculate relative levels of illumination..