A photographic lens for which the focus is not adjustable is called a fixed-focus lens. The focus is set at the time of manufacture, and remains fixed. It is usually set to the hyperfocal distance, so that the depth of field ranges all the way down from half that distance to infinity, which is acceptable for most cameras used for capturing images of humans or objects larger than a meter.
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Concept
In order to reach a short minimal focal distance the aperture and the focal length of the lens are reduced, so that the hyperfocal distance is small. This allows for the depth of field to be extended from infinity to a short distance.
The disadvantage is the reduction of light that will reach the film through the small aperture. Therefore the lenses are usually not suitable for fast-moving objects which require short exposure times. The amount of collected light can be increased by opening the angle of view, which is achieved with an even shorter focal length resulting in a wide-angle lens. Telephoto lenses are not feasible at a reasonable lens speed.
Applications
Fixed focus can be an inexpensive alternative to autofocus, which requires electronics, moving parts, and power. Since a fixed-focus lens requires no input from the operator, it is suitable for use in cameras designed to be inexpensive, or to operate without electrical power as in disposable cameras, or in low-end 35 mm film cameras, or in cameras featuring simple operation.
Especially suitable are fixed focus lenses for low resolution CCD cameras as found in webcams, surveillance cameras and mobile phones, because the low resolution of the detector allows a loose focusing on the CCD without noticeable loss of image quality. Therefore the circle of confusion gets bigger and hyperfocal distance smaller.
Special-purpose cameras such as the Agiflite[1] are used for situations like aerial photography from aircraft. Because the ground is far from the camera, focus adjustment is not necessary. For 35 mm cameras, some superwide fixed-focus lenses have been made.[2]
See also
References
- ^ "Agiflite Aerial camera". Photo.net BBoard. http://photo.net/bboard/q-and-a-fetch-msg?msg_id=004pdu.
- ^ "The Strange Case of the 12mm Fisheye". TheSyberSite. http://thesybersite.com/general/12mm-fisheye/index.htm.
External links
This article was originally based on "Fixed focusing" in Camerapedia, retrieved at an unknown date under the GNU Free Documentation License.
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