When choosing a new lens, or considering a camera with one built into it, the main consideration we have is its focal length. How much we can fit into a image, and how far we can zoom into distant details are determined by this, and together with its aperture it determines what types of photography are available to us.
Depth of field.
In optics, particularly as it relates to film and photography, depth of field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. Although a lens can precisely focus at only one distance at a time, the decrease in sharpness is gradual on each side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions.Rack focus
A rack focus in filmmaking and television production is the practice of changing the focus of the lens during a shot. The term can refer to small or large changes of focus. If the focus is shallow, then the technique becomes more noticeable. In professional films, a camera assistant called a focus puller is responsible for rack focusing. In his documentary film, The Sinister Saga of Making "The Stunt Man" the director Richard Rush claims he developed the technique in the 1960s. In the commentary to Firefly, Visual Effects Supervisor Loni Peristere comments that rack focus has been taboo in recent cinematography because such shots cannot be "repaired" in post production.
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