Using depth of field, you can blur objects that are outside the camera's focal range. The further an object is from the focal range, the more it is blurred.
(a) Near sharpness plane (b) Focal range (c) Far sharpness plane (d) Motion path for point of interest (e) Camera point of interest (f) Motion path for camera eye (g) Camera eye
The Target camera uses depth of field and clearly indicates its focal range when in Top, Side, or Front view. With the Free camera, you can change the field of view by Alt-dragging the near sharpness plane. This changes the camera only; what you are viewing remains the same size.