Fixed focus lens
Encyclopedia
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 image
s. Most camera
s 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.
), 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 lens
es 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-up
s, 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 portrait
s, 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.
, 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
s, or in low-end 35 mm film point and shoot camera
s, or in cameras featuring simple operation. These are usually wide-angle lens
es 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 webcam
s, surveillance cameras and camera phone
s, 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
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.
Photographic lens
A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.While in principle a simple convex lens will suffice, in...
for which the focus
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...
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
Hyperfocal distance
In optics and photography, hyperfocal distance is a distance beyond which all objects can be brought into an "acceptable" focus. There are two commonly used definitions of hyperfocal distance, leading to values that differ only slightly:...
, so that the depth of field
Depth of field
In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...
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
Depth of field
In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...
to produce acceptably sharp image
Image
An image is an artifact, for example a two-dimensional picture, that has a similar appearance to some subject—usually a physical object or a person.-Characteristics:...
s. Most camera
Camera
A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...
s with focus free lenses also have a relatively small aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
which increases the depth of field. Fixed-focus cameras with extended depth of field (EDOF) sometimes are known as full focus cameras.
Concept
In order to reach a short minimal focal distance the aperture and the focal length of the lens are reduced (a slow wide-angle lensWide-angle lens
From a design perspective, a wide angle lens is one that projects a substantially larger image circle than would be typical for a standard design lens of the same focal length; this enables either large tilt & shift movements with a view camera, or lenses with wide fields of view.More informally,...
), so that the hyperfocal distance is small. This allows for the depth of field
Depth of field
In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...
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
Lens speed
Lens speed refers to the maximum aperture diameter, or minimum f-number, of a photographic lens. A lens with a larger maximum aperture is a fast lens because it delivers more light intensity to the focal plane, allowing a faster shutter speed...
. The amount of collected light can be increased by opening the angle of view
Angle of view
In photography, angle of view describes the angular extent of a given scene that is imaged by a camera. It is used interchangeably with the more general term field of view....
, which is achieved with an even shorter focal length resulting in a wide-angle lens
Wide-angle lens
From a design perspective, a wide angle lens is one that projects a substantially larger image circle than would be typical for a standard design lens of the same focal length; this enables either large tilt & shift movements with a view camera, or lenses with wide fields of view.More informally,...
. Telephoto lens
Telephoto lens
In photography and cinematography, a telephoto lens is a specific type of a long-focus lens in which the physical length of the lens is shorter than the focal length. This is achieved by incorporating a special lens group known as a telephoto group that extends the light path to create a long-focus...
es are not feasible at a reasonable lens speed
Lens speed
Lens speed refers to the maximum aperture diameter, or minimum f-number, of a photographic lens. A lens with a larger maximum aperture is a fast lens because it delivers more light intensity to the focal plane, allowing a faster shutter speed...
.
The advantage of this design is that it can be produced very inexpensively, more so than autofocus
Autofocus
An autofocus optical system uses a sensor, a control system and a motor to focus fully automatic or on a manually selected point or area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system has to be done manually until indication...
or manual focus
Manual focus
In the field of photography, a manual focus camera is one in which the operator has to adjust the focus of the lens by hand. Before the advent of autofocus, all cameras had manually adjusted focusing; thus, the term is a retronym....
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-up
Close-up
In filmmaking, television production, still photography and the comic strip medium a close-up tightly frames a person or an object. Close-ups are one of the standard shots used regularly with medium shots and long shots . Close-ups display the most detail, but they do not include the broader scene...
s, 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 portrait
Portrait photography
Portrait photography or portraiture is the capture by means of photography of the likeness of a person or a small group of people , in which the face and expression is predominant. The objective is to display the likeness, personality, and even the mood of the subject...
s, 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.
Applications
Fixed focus can be a less expensive alternative to autofocusAutofocus
An autofocus optical system uses a sensor, a control system and a motor to focus fully automatic or on a manually selected point or area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system has to be done manually until indication...
, 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
Disposable camera
The disposable or single-use camera is a simple box camera sold with a roll of film installed, meant to be used once. Most use fixed-focus lenses. Some are equipped with an integrated flash unit, and there are even waterproof versions for underwater photography...
s, or in low-end 35 mm film point and shoot camera
Point and shoot camera
A point-and-shoot camera, also called a compact camera, is a still camera designed primarily for simple operation. Most use focus free lenses or autofocus for focusing, automatic systems for setting the exposure options, and have flash units built in....
s, or in cameras featuring simple operation. These are usually wide-angle lens
Wide-angle lens
From a design perspective, a wide angle lens is one that projects a substantially larger image circle than would be typical for a standard design lens of the same focal length; this enables either large tilt & shift movements with a view camera, or lenses with wide fields of view.More informally,...
es with fixed aperture, and cameras with these lenses generally use a viewfinder
Viewfinder
In photography, a viewfinder is what the photographer looks through to compose, and in many cases to focus, the picture. Most viewfinders are separate, and suffer parallax, while the single-lens reflex camera lets the viewfinder use the main optical system. Viewfinders are used in many cameras of...
for composition.
Especially suitable are fixed-focus lenses for low resolution CCD cameras as found in webcam
Webcam
A webcam is a video camera that feeds its images in real time to a computer or computer network, often via USB, ethernet, or Wi-Fi.Their most popular use is the establishment of video links, permitting computers to act as videophones or videoconference stations. This common use as a video camera...
s, surveillance cameras and camera phone
Camera phone
A camera phone is a mobile phone which is able to capture still photographs . Since early in the 21st century the majority of mobile phones in use are camera phones....
s, 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
Circle of confusion
In optics, a circle of confusion is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source...
gets bigger and hyperfocal distance smaller.
Special-purpose cameras such as the Agiflite
are used for situations like aerial photography
Aerial photography
Aerial photography is the taking of photographs of the ground from an elevated position. The term usually refers to images in which the camera is not supported by a ground-based structure. Cameras may be hand held or mounted, and photographs may be taken by a photographer, triggered remotely or...
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.