The aperture control on a camera controls the opening in the lens curtain, precisely measuring the amount of light allowed through the camera lens and into the camera. Wide apertures have low "f" numbers and allow more light; narrow apertures have high "f" numbers and allow less light to enter. Changing the aperture changes the amount of light and affects the focus plane of the image, allowing for artistic effects. Changing the aperture on a Canon SLR begins with selecting the correct shooting mode.
Select a shooting mode where the aperture is an adjustable factor. Many automatic modes and some semi-automatic modes choose the aperture setting for you; if you are shooting in one of these modes, you will not be able to change the aperture. Select either fully manual (M) or aperture priority (Av) mode in order to free the aperture controls.
In aperture priority mode, use the dial located at the top left of the camera body to change the aperture setting. This dial is located just behind the shutter release button. Look through the view finder or on the top LCD display to see your current aperture settings and adjust them according to your preference.
In fully manual mode, turn the function switch located near the bottom right corner of the LCD screen to select the top level of functions. Once the top level of function is selected, use the dial to the right of the switch to select your desired aperture. This will allow you to select your aperture, which is then displayed by looking at the LCD display or through the view finder.