# Fixed-focus lens

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[[File:Back of iPhone 3G white showing the fixed focus 2 megapixel camera.jpg|thumb|The back of an [iPhone 3G](/source/iPhone_3G), showing its small fixed-focus lens]]
[[File:Kodak Duaflex IV.jpg|thumb|upright|Mid-20th-century [medium-format](/source/Medium_format_(film)) fixed-focus camera]]

A [photographic lens](/source/photographic_lens) for which the [focus](/source/Focus_(optics)) is not adjustable is called a '''fixed-focus lens''' or sometimes '''focus-free'''. The focus is set at the time of lens design, and remains fixed. It is usually set to the [hyperfocal distance](/source/hyperfocal_distance), so that the [depth of field](/source/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.

Rather than having a method of determining the correct focusing distance and setting the lens to that focal point, a fixed-focus lens relies on sufficient [depth of field](/source/depth_of_field) to produce acceptably sharp images. Most [camera](/source/camera)s with focus-free lenses also have a relatively small [aperture](/source/aperture), which increases the depth of field. Fixed-focus cameras with extended depth of field (EDOF) sometimes are known as '''full-focus''' cameras.

== Concept ==
{{Unreferenced section|date=March 2012}}
In order to reach a minimal focal distance, the aperture and the focal length of the lens are reduced (a slow [wide-angle lens](/source/wide-angle_lens)), to make the hyperfocal distance small. This allows the [depth of field](/source/depth_of_field) to extend from a short distance to infinity.

A disadvantage of this is there will be reduction in light that will reach the film or sensor through the small aperture, since the narrower opening lets less light in. That means that these types of lenses are usually not suitable for fast-moving objects which require short exposure times to freeze motion – see [lens speed](/source/lens_speed). The amount of collected light can be increased by opening the [angle of view](/source/angle_of_view), which is achieved with an even shorter focal length resulting in a [wide-angle lens](/source/wide-angle_lens). [Telephoto lens](/source/Telephoto_lens)es are not feasible at a reasonable [lens speed](/source/lens_speed).

The advantage of this design is that it can be produced very inexpensively, more so than [autofocus](/source/autofocus) or [manual focus](/source/manual_focus) systems. The system is also effectively automatic; the photographer need not worry about the focal point for a given scene. Fixed-focus lenses are unable to produce sharp [close-up](/source/close-up)s, or images of objects that are only a fraction of the hyperfocal distance from the camera which, depending on factors including the size of the camera, may be within {{convert|2.4|–|3.7|m|ft|abbr=off|sp=us}}.

== Applications ==

Fixed focus can be a less expensive alternative to [autofocus](/source/autofocus), which requires electronics, moving parts, and power. Since fixed-focus lenses require no input from the operator, they are suitable for use in cameras designed to be inexpensive, or to operate without electrical power as in [disposable camera](/source/disposable_camera)s, or in low-end 35 mm film [point and shoot camera](/source/point_and_shoot_camera)s, or in cameras featuring simple operation. These are usually [wide-angle lens](/source/wide-angle_lens)es with fixed aperture, and cameras with these lenses generally use a [viewfinder](/source/viewfinder) for composition.

Especially suitable are fixed-focus lenses for low-resolution [CCD camera](/source/CCD_camera)s as found in [webcam](/source/webcam)s, [surveillance cameras](/source/surveillance_cameras) and [camera phone](/source/camera_phone)s, because the low resolution of the [image sensor](/source/image_sensor) allows a loose focusing on the CCD without noticeable loss of image quality. This makes a bigger [circle of confusion](/source/circle_of_confusion) and smaller hyperfocal distance.

Special-purpose cameras such as the Agiflite<ref>{{cite web 
| url = http://photo.net/bboard/q-and-a-fetch-msg?msg_id=004pdu 
| title = Agiflite Aerial camera 
| work = Photo.net BBoard}}</ref> 
are used for situations like [aerial photography](/source/aerial_photography) from aircraft. Because the ground is far from the camera, focus adjustment is not necessary. [Light-field camera](/source/Light-field_camera)s use fixed-focus lenses, since the focus can be set when the picture is displayed. For 35&nbsp;mm cameras, some superwide fixed-focus lenses have been made.<ref>{{cite web | title = The Strange Case of the 12mm Fisheye | url = http://thesybersite.com/general/12mm-fisheye/index.htm | work = TheSyberSite }}</ref>

==See also==
* [Autofocus](/source/Autofocus)
* [Prime lens](/source/Prime_lens)

==References==
{{reflist}}

Category:Photographic lenses

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Adapted from the Wikipedia article [Fixed-focus lens](https://en.wikipedia.org/wiki/Fixed-focus_lens) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Fixed-focus_lens?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.