What is the Aperture

Lens aperture showing the variable amount of lens opening

The aperture is an opening in the lens that allows a variable amount of light to travel through the lens and onto the image sensor.  The aperture is therefore one of the  methods of controlling the exposure of the image. The f-stop of the aperture is a number that indicates the size of the opening, and as the F-stop number increases in size, the aperture hole becomes smaller.  In other words, a large F-stop number has a small opening and a small F-stop number has large opening.  Opening the aperture in increments from f/16 to f/11 and f/11 to f/8 and so on increases the amount of light by twice the amount.  By the same token, changing the f-stop from f/8 to f/11 decreases the amount of light by half.  The opening or closing of the aperture in these increments are called "Stops", so for instance, if you camera was set for f/16, and someone said you have to open up the aperture by "two stops", you would end up at f/8.  Most modern cameras also allow for half or one third stop increments.

The f-stop range can differ amongst lenses, for instance, the range of can be as follows: f/1, f/1.4,f/2,f/2.8,f/4, f/5.6, f/8, f/11, f/16, f/32, f/45, f/64, f/90, f/128 etc.  A lens with this many stops would be rare, very expensive, and probably wouldn't fit your camera, unless you equally special camera.  When purchasing a camera lens for cameras with the capability of changing lenses, it is preferable to buy the fastest lens possible (the smallest aperture number.)  This will allow you to capture images in low light, or use a faster shutter speed.

The aperture also controls "Depth-of-Field" or DoF.  DoF is a distance in front and behind the subject where the image will be in focus.  The distance from the camera to the subject and the focal length of the lens will also determine how much Depth-of-Field will be available in the.  The subject of Depth-of-Field with be covered later in a separate section.