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One secret to good photography is isolating the subject from a distracting or unnecessary background. In the studio, the photographer simply picks whatever backdrop they feel best suits the portrait, but when shooting outdoors or on location, you frequently have to find other ways create pleasing backgrounds. One of the most tried and true methods is to use a technique known as "selective" focus. This takes advantage of the limited depth of field available to longer focal lengths used at maximum aperture. Shooting with a 300 2.8 lens wide open and close enough to the subject for a head and shoulders shot will produce a sharp image of the face and eyes but usually throw the background completely out of focus so long as there's at least a little distance between the subject and background.
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All the photos shown above use some form of selective focus to clean up distracting backgrounds (click on any image to enlarge). In each case a longish focal length was used (except for the shot of Lisa at the Vietnam Memorial) to minimize depth of field coupled with a large lens opening to reduce depth of field even further.
The shot of Lisa shows an interesting possibility of the Lensbaby Composer, namely the ability to use selective focus with image elements that are the same distance from the camera. In this shot, the real Lisa is thrown out of focus while her reflection is in focus (an earlier shot reversed this effect) making it possible to force the eye to consider the reflected image of Lisa as the main point of the photograph. This technique is not possible with regular camera lenses although it can be done other ways.
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Using this technique
The basic rule of thumb for selective focus is the longer the focal length, the larger the lens opening and the closer the lens is to the subject and the greater the distance between the subject and the background, the greater the out of focus blur effect. This usually preferences telephoto lenses which offer less inherent depth of field than shorter focal length lenses. Wide angle lenses can't produce the selective focus effect per se, but since they can exaggerate the distance between foreground and background elements can still be used to produce cleaner backgrounds. Another alternative to telephoto lenses for selective focus is use of something like a Lensbaby selective focus lens. These optics, which can take wide angle and super wide angle add on lenses can give the photographer both a wide field of view and selective focus. This tool also gives the photographer an option even telephoto lenses don't offer, the ability to put one subject in focus and other out of focus when both are on the same plane. Regular lenses can only achieve this effect when image elements are not on the same plane. Selective focus is a very useful technique that should be in every photographer's repertoire. It's especially helpful when you're photographing people outdoors and want to get rid of distracting backgrounds.