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Scale focus, or zone focus, is a type of focusing system used by many inexpensive cameras from the 1940s and 1950s. These cameras have an adjustable focus, but lack a focusing aid such as a rangefinder. It is necessary to determine the distance to the subject and set the focus using a scale printed on the lens. If you are good at estimating distances, or have a tape measure at hand, you can get precise, sharp focus with one of these cameras.
Technique
Today the term "zone focus" refers to the technique of setting a fixed focal distance (turning off autofocus), and often a fixed aperture, hence fixing the depth of field, and then taking photographs at that distance; the DOF can be read off of the DOF scale on the focus ring, hence the term "zone". This is frequently used in street photography to allow rapid and candid "shooting from the hip" (shooting without composing in the viewfinder) and avoiding autofocus lag. Zone focusing is also useful for pet action photography, where the time delay of focusing may be unworkable.