Aperture is the control of how wide the opening is in the lens of the camera. It also affects how much light is hitting the sensor. A simple way to think of Aperture is a faucet of running water. If the faucet is wide open then you get a lot of water. If it is almost closed then it is just a trickle. This is the same for light coming through the camera lens. One of the main purposes of the aperture is to control depth of field (see previous blog post on Depth of Field). The size of the opening is recorded in what is call an f-stop. Now, the math is a bit different for the Aperture than the shutter speed but the principle is the same. Each step represents the doubling or halving of the amount of light hitting the sensor. The following is a general list of the most common apertures: 2.8, 4, 5.6, 8, 11, 16, 22. The smaller the number the larger the opening in the camera and thus the shallower depth of field. When you hear the term "fast lens" the aperture is what they are referring too. Why? Well, if you have a wide open lens (say 2.8) then the amount of time the shutter has to be open to make a proper exposure is shorter than it would be with a smaller aperture (larger number). This means that if you are taking photos in lowlight you can open up your lens and raise your shutter speed. When you are photographing wildlife in the early morning you need a fast lens. If you are photographing a wedding inside a dimly lit church when no flash photograph is allowed then you need a really fast lens. The first image below shows a very small aperture (high f-stop number). The second shows a wide open aperature (small f-stop number). You can clearly see the differance in depth of field.
Exposure: 0.7sec at f/16
Exposure: 1/25 sec at f/3.2