Focus is fundamental to photography, but something we tend to take for granted in everyday life - unless we wear glasses that is. For something to be in focus on a photograph the camera has to be adjusted so that beams of light coming from a common point on the subject are projected onto the film at a correspondingly common point. This is achieved mechanically by altering the distance between the lens and the film. In today's auto focus cameras the process is generally ignored, but in older types it's very much part of the picture making process. In those cameras fitted with reflex viewing or ground glass screens, focus is achieved by inspecting the image, the resulting photograph will exactly match the photographer's point of focus. With cameras without these features focus is achieved by reference to a scale with distance marked. The distance refers to a measurement taken from the FILM PLANE to the subject. Generally this distance was guessed, with practice it's possible to become quite accurate. The technique I use is to imagine how many times my height goes into the distance between me and the subject. Over about 30ft accuracy ceases to be so important. However close up photography requires great accuracy of distance measurement, and remember the measurement is taken from the back of the camera, not the lens. Two photographs below at 3.5ft, one measured correctly and the other focussed at 2ft.
Two common methods of setting focus are a scale on the baseboard of bellows type cameras or a scale around the outside of the lens mount which moves in or out on a screw thread as it is rotated, the focus point being read off against a reference point. A third, less common, method is the use of a knob somewhere on the body. Three examples below.
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