Blurred backgrounds isolate and accent your photographic subject, as well as provide a sense of depth, a third dimension to a flat image. Sharpness and blur are functions of focus and aperture, which is the size of the opening that allows light into a camera. Together, these are responsible for the effect of depth of field. Understanding how depth of field works will help you control your photographs and selectively blur your backgrounds.
Depth of Field
When a camera is focused on a point, its image is sharp on the plane of the camera's sensor. Objects in front and behind are obviously out of focus to varying degrees, but it isn't clear where the focus begins and ends. In fact, there is only one precise plane of focus and all points in front and behind become increasingly out of focus. Depth of field refers to the range in which these out-of-focus points appear acceptably sharp.
Point of Focus
Depth of field is not a fixed distance. It has a relationship to the plane of focus both in front and behind. When the subject is close to the camera, the depth of field is shorter in real distance, and much longer when the subject is more distant. Getting close to your subject places more background elements outside the depth of field range behind the point of focus. This increases background blur, no matter what type of camera you use.
A good way to envision the effect that aperture has on blur is to imagine a cone originating at the camera's aperture, representing the fall-off of focus behind your subject. When the aperture is small, the cone is narrow. More of the background is within range of acceptable sharpness. Using a wide aperture, the cone is wide, so objects behind, and closer to, the subject appear out of focus. Apertures are expressed as f-stops, a ratio of the opening to the lens focal length. A small aperture has a high f-number, such as f16 or f22, while a wide aperture has a small number, such as f1.4 or f2.8. To blur backgrounds, the smaller the f-number, the better.
Controlling aperture settings is possible only on more expensive cameras, however most digital cameras, regardless of price range, have built-in settings. Portrait and Close-up settings are two programs that put cameras in conditions to maximize background blur. Try both settings and then get as close as you can to the subject to improve the blurred background effect. Dim light also causes camera programs to automatically open the aperture wider, so shooting in shade blurs backgrounds more readily than bright sun.
Strictly speaking, a telephoto or zoom lens doesn't change the way depth of field operates, but it does magnify blurry elements in the background, essentially doing the same thing. Using a long focal length lens or the maximum zoom length may give you exactly the blur you want. Point-and-shoot cameras with digital zoom, however, use only part of the camera's image sensor, so resulting photos are lower resolution than those taken with optical zoom only.
- Photo Credit Ronnie Kaufman/Larry Hirshowitz/Blend Images/Getty Images
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