Photography's root comes from the Greek roots "photo" which means light, and "graph" which means to write or scribe. Photography is all about light and how to interact with it. Two basic functions of an SLR camera accomplish this through the aperture and shutter speed.

{mosgoogle}Shutter speed refers to the amount of time that the film, or in the case of a digital SLR, how long the image sensor is exposed to light. Depending on the camera, shutter speeds can vary from 30 seconds to 1/8000 of a second. Some cameras have a 'bulb' mode that allows the user to open and close the shutter manually using a switch.

The aperture setting of a camera refers to the amount of light that is let into the camera body by the lens. An iris contracts or expands depending on this setting on the camera, thus controlling the light that hits the film. Aperture terminology may be confusing as the "higher" the aperture, the lower its number. Lenses are rated by their aperture and focal length. For instance, Canon might have an F/2.8 28-75mm lens. The F/ prefix is the highest aperture that the lens can produce. In this case, an aperture of 2.8 is the maximum amount of light that the lens can let in. A lens that has a high aperture is considered a "fast" lens.

Shutter speed and aperture values are at a constant tug-of-war concerning light. Suppose someone was a handed a camera and was told that its aperture and shutter settings were set to ensure proper exposure using a shutter speed of 1/60th and an aperture of F/8. However, suppose that they wanted a faster shutter speed to capture a moving object, so they set the shutter speed to 1/250 and took a picture, they would get an underexposed photo because they didn't allow the film to be exposed for the 1/60th of a second it needed. Therefore, they would need to change the aperture to allow more light in, perhaps by changing it to F/1.8, or whatever the maximum aperture of the lens is. It is always good once you have changed your settings to check them with a light meter and see if you are giving enough light for proper exposures.