Prior to the advent of rangefinders and reflex focusing systems in 135 and medium format cameras, guess focusing was the most common way of focusing lens. It also served as an alternative to ground glass focusing.
Users estimated the distance to the subject and set a focusing ring or focusing lever to the correct distance. Often, the method made use of depth of field scale markings on the lens barrel.
Usually the lens on guess focusing cameras were fixed. Their apertures were not too wide, typically f/2.8 or f/3.5 at most. It was considered too difficult to guess the distance with enough precision to produce sharp images at wider apertures with shallower depth of field.
A variant of guess focusing was scale focusing or zone focusing. Symbols, such as one person, a group of people and a mountain, on the focusing ring indicated typical distances for picture taking depending on the subject.