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Fixed focus

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Glossary Terms

A camera with a fixed focus (or "panfocus") lens has no focus control. The focus is preset by the designer, usually at a distance for which depth of field will reach infinity even at the greatest aperture possible with the lens.

Depth of field decreases with focal length and aperture. As the focal length of a "standard" lens decreases with the frame area, a fixed-focus camera for 127 or 35mm film is typically more useful than (because it has a greater depth of field than) one for 120 film.

Fixed focus is generally only used on the cheapest cameras of any period. Box cameras are usually fixed-focus, as are the cheapest toy cameras. Traditionally the next rung above fixed focus has been guess focusing (zone focusing).

As autofocusing logic, circuitry and mechanisms became better and cheaper, autofocus models seem to have largely supplanted fixed-focus cameras. However, fixed-focus cameras survived in the marketplace for a long time, thanks in part to the improvement of emulsions for fast films and to small, built-in electronic flashguns. As an example, the Fuji Hi! Mickey Mouse camera of 1995 has such a built-in flashgun, an 33mm f/8 lens;[1] the hyperfocal distance for such a lens is about 4.6m and if this is indeed its fixed focus then at f/8 everything from 2.3m to infinity is in focus (at f/16, everything from 1.52m).[2]

The cameras built into cheaper mobile phones are fixed focus (although they may add a "macro" switch for two focusing distances rather than one).

Other fixed-focus cameras include those for prison mug-shots. The MPP Prison camera, for example, has a fixed-focus 135mm or 150mm lens for two exposures on 4×5 in. sheet film.[3]

NotesEdit

  1. Koyasu, pp. 55–6.
  2. These calculations assume a maximum tolerable circle of confusion of 0.03mm and ignore diffraction; they are made at the DOFMaster depth of field calculator.
  3. Skinner, pp. 45–6. The 150mm Schneider lens on the example Skinner presents has a maximum aperture of f4.5; unfortunately we do not learn its fixed distance.

ReferencesEdit

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