Canon's EOS 5D Mark II, a digital single-lens reflex camera, offers photographers complete control over their images with a fully manual shooting mode. In this mode, photographers must set both the shutter speed and the aperture. While the shutter speed dictates the amount of time an image is exposed, the aperture controls the amount of light coming through the lens. The wider the lens opening, the smaller the aperture number, also called an f/stop. The aperture is set on the 5D using the "Quick Control Dial."
Set the camera to "M" using the Mode dial on the top-left side of the camera.
Turn the "Power" switch to the icon shaped like a "7" laying on its side. This will allow you to use the "Quick Control Dial" to change the aperture setting.
Twist the Quick Control Dial, which is the large dial to the right of the LCD screen, to change the aperture setting. You will see the aperture number displayed in the upper-right corner of the small display panel on the top-right side of the camera.
Set the corresponding shutter speed using the Main Dial, found above the shutter button.
The lens aperture also controls the depth of field of a picture, or how sharp or fuzzy the background appears. A wide aperture allows more light through the lens and creates a shallow depth of field. The smaller the aperture, the sharper the background will appear. If you're unsure about the correct aperture and shutter speed combination, try bracketing. Take a series of pictures of one scene in sets, changing the aperture or shutter speed one position for each frame. You will see the entire range of exposure and learn which settings work best in a particular environment. Bracketing the aperture will allow you to experiment with the depth of field, seeing whether an image looks better with a sharper or fuzzier background.