Selective Focus is the practice of using large apertures and/or telephoto or macro lenses to greatly restrict the area of focus in a photo (or in a scene or shot in a motion picture). The purpose is to separate the subject of the image from the background (or foreground - or both).
This technique takes advantage of a neurological adaptation of the human brain called visual attention. When humans are presented with visual information the human eye (directed by the brain) will automatically be drawn to information that is focused and sharp when presented against information that is not focused.
Much like the movement of an object in our periphery will draw our attention, selective focus can be used to control the composition of a photo drawing the viewer's eye to the portion of the framed subject the photographer feels is most important.
The decision by the photographer to draw attention to only one part or one subject in a complex multi-subject composition gives the photographer an enormous amount of control over how the information in the photo is perceived by the viewer. Changing focus from one subject to another can completely change the tone, the emotion, the context of an image.
Photography is a unique form of creative expression in that it is technology-based and relatively complex (the underlying math and science required to make a camera and lens system work will give any truly creative right-brained individual a migraine). Yet as a creative outlet photography continues to grow with each passing year.
Selective focus is one of the key creative concepts floating in the sea of physics and math that makes capturing light particles and translating them into viewable images possible. Selective focus is the main tool that creative photographers can always count on to elevate the technical aspects of the medium into something that has emotional impact and is expressive.
The practice of "pulling focus" in a series of photographs illustrates how two photos taken at the same location with the same orientation can have two very different outcomes when the focal point for each is shifted; this shift in focus changes the main subject of the image.
As a photographer or as a cinematographer - composing the shot - choosing the main focal point and thus the main subject of the shot - can provide an enormous amount of creative flexibility.
For the photographer the decision is often "what part of the action in front of me is most interesting or best expresses my viewpoint?"
Small changes in focus - such as changing the focus from the drum and the back of the drummer to the face of the bassist (Lucky Chucky on drums and Dirty Ray on bass - Bret Michaels Band) - can provide more emotional impact for the viewer of the image to experience.
Other times - such as in the photo below - you can take an ordinary item and make it the star of the image simply by letting the focus drop off everywhere in the image but where that item sits.