When a camera lens is focused on a subject, there is only perfect focus at one particular distance. Anything in front of or behind that subject will be out of focus to a greater or lesser degree depending on several factors. Depth of Field is the portion of the photograph that is in reasonably sharp focus.
A photographer can use depth of field as a creative tool to draw the viewer’s eye to what he wants to convey in the picture. When photographing a person, you may want to throw the background out of focus so that the subject’s face is the most prominent feature. On the other hand, if you are photographing a landscape, you probably want everything from the foreground to the horizon to be in focus.
One way a photographer can achieve the desired effect is to change the aperture setting on the camera. Aperture refers to the size of the opening in the lens through which light passes. It is measured in something called F-stops and written in numbers such as 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 and 22. Aperture can be adjusted to allow a wide beam of light, which creates a shallow (less) depth of field, or a narrow beam of light which creates a greater (more) depth of field. The tricky thing here to remember is that the smaller the f-stop number, the bigger the opening, the larger the f-stop number, the smaller the opening. This seems completely backwards but an easy way to remember is:
Small number – Small depth of field
Large number – Large depth of field
Most of today’s digital SLRs will have an aperture priority mode. This setting allows the photographer to control the aperture while the camera automatically sets the correct shutter speed. Remember though, when you look through the viewfinder, the lens is always open to its largest aperture, which is giving you a shallow depth of field. It is only when the shutter release button is pressed that the aperture will stop down to the preset exposure. Many cameras have a depth of field preview button. After you focus on the scene, you can press the preview button to see the scene as the sensor will see it.
The photographs below show the same scene using different aperture settings. Notice the outer cards as the aperture gets smaller (larger F-number) – they become less blurred.
The focal length of the lens and distance from camera to subject also affect depth of field. A wide-angle lens will give you extensive depth of field and make it easier to keep everything in the scene sharp. If you are aiming to blur your background, either use a telephoto lens or move closer to your subject.
To get maximum depth of field and have as much of the scene as sharp as possible, use a wide angle lens, set a small aperture and stand back from your subject.
To get less depth of field, or a blurred background, use a telephoto lens, set a large aperture and get close to your subject.
- New York Institute of Photography
- Aperture diagram obtain from Wikimedia Commons under GFDL