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.
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 to produce acceptably sharp images. Most cameras with focus free lenses also have a relatively small aperture which increases the depth of field. Fixed-focus cameras with extended depth of field (EDOF) sometimes are known as full focus cameras.
Contents |
In order to reach a short minimal focal distance the aperture and the focal length of the lens are reduced (a slow wide-angle lens), 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 – see lens speed. 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.
The advantage of this design is that it can be produced very inexpensively, more so than autofocus or manual focus systems. The system is also effectively automatic; the photographer need not worry about focusing. It can also be more predictable than automatic systems.
Disadvantages include the inability to produce images as sharp as a lens that has been set to the best focal point for a given scene. Fixed-focus lenses are unable to produce sharp close-ups, or images of objects close to the camera, usually within 2.4 - 3.7 meters (8–12 feet). The latter limitation makes them unsuitable for portraits, as they cannot fill the frame of an image with a person's face and render it sharp at the same time. This limitation is likely to confuse inexperienced photographers.
Fixed focus can be a less expensive alternative to 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 cameras, or in low-end 35 mm film point and shoot cameras, or in cameras featuring simple operation. These are usually wide-angle lenses with fixed aperture, and cameras with these lenses generally use a viewfinder for composition.
Especially suitable are fixed-focus lenses for low resolution CCD cameras as found in webcams, surveillance cameras and camera 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]
This article was originally based on "Fixed focusing" in Camerapedia, retrieved at an unknown date under the GNU Free Documentation License.