The background of an image is all too often overlooked. It is easy to concentrate exclusively upon the subject and forget to consider the merits of the background before releasing the shutter. In landscape work the background may be the subject, and everything may be rendered in sharp focus, but in portraiture it more likely that the subject will be in sharp focus in the foreground and the background must be made of little consequence. Certainly, the background in a head and shoulders portrait, for example, should not include any element of detail or colour which might distract the viewer's eye from the person. We have all seen photographs of people where an unnoticed fire-extinguisher seems to project from the top of the subject's head!
As always in photographic composition, it is up to the photographer to get things right from the start. When looking through the viewfinder and at the subject, be sure to scan your eye around each of the corners of the frame in search of anything distracting. Distractions might take the form of in-focus detail, areas of bright colour, bright areas of sky, strong lines, significant variations of tone, specular or diffuse reflections etc. Where some background element seems to be distracting, try taking a step sideways so that the offending detail moves out of frame or behind the subject. After a while, such actions become second nature.
Other considerations include keeping a human subject at least a few metres away from a background, so making it easier to use differential focus and shallow depth of field, and taking care when using flash or fill-flash to avoid unsightly background shadows and unwanted reflections.
In general, the background should remain subordinate to the subject or subjects in terms of detail, tone and interest. It is a lack of interest or detail that makes the foreground subject stand out and grab attention. How this is achieved is a matter for the photographer and will depend upon the particular circumstances, but the most commonly used technique is careful control of shallow depth of field. The resulting background softness renders detail indistinguishable. It softens and blurs, and tends to reduce variations of tone and texture to a uniform blur. All the photographer has to do is determine what depth of field is required to render the subject sharp. The depth of field envelope can then be set appropriately and positioned in an optimum manner by focusing about one third of the distance in to the depth occupied by the subject. For example, where the subject extends from three to six metres from the camera, focus at about four metres while using an aperture that provides an appropriate depth of field.
Moving subjects can be rendered sharp against a blurred background by panning the camera with the subject's movement and selecting an appropriate shutter speed.
In cases where the background is the subject, or at least an essential element of the subject, it is usually important to render it in sharp focus. It may be possible to use a small aperture and a slow shutter speed to render the whole image sharp, but other approaches are also possible. Out-of-focus backgrounds and foregrounds focus attention sharply on an in-focus middleground subject picked out using differential focus, and flowing water might be used as a soft blurred path linking a sharp foreground and background.
In the rather unusual case of the image on the left, the nude model is left out of focus despite arguably being the subject of the image. The shoes in the foreground are rendered sharp. The possibilities are limited only by the imagination of the photographer.