Fixed focus
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. Such cameras are sometimes labeled with the euphemism "focus free," but this should not be confused with true autofocus.
Fixed-focus cameras typically restrict their widest aperture to f/8 or smaller, to yield adequate depth of field. Depth of field decreases with longer lens focal length, and the focal length of a "standard" lens scales in proportion to the image-format dimensions. So a fixed-focus camera for 35mm film will give 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 often fixed-focus, as are the cheapest toy cameras. The next level of control is two-zone focus, by use of a built-in auxiliary lens (e.g. to give 'portrait' range with better box cameras). Slightly better cameras have zone focusing (a full focus range, but marked only in zones, typically for a single portrait, a group, and a landscape) or scale focusing, with the actual focus distance marked.
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]
Notes
- ↑ Koyasu, pp. 55–6.
- ↑ 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.
- ↑ 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.
References
- Koyasu Yoshinobu (子安栄信). "Fuji Shashin Firumu no kamera no subete" (富士写真フィルムのカメラのすべて All the cameras of Fuji Photo Film). In Kamera Rebyū: Kurashikku Kamera Senka (カメラレビュー クラシックカメラ専科) / Camera Review: All about Historical Cameras no.44, December 1997. ISBN 4-257-13013-X. Tokushū: Fuji Shashin Firumu no kamera (特集：富士写真フィルムのカメラ, special issue on the cameras of Fuji Photo Film). Pp.18–91.
- Skinner, Basil. Micro Precision Products: The MPP Story and the Products. Newquay, Cornwall: MPP Publications, 2004. ISBN 0-9546070-1-5
