Camera lens

A camera lens, photographic lens or photographic objective is an optical

.

There is no major difference in principle between a lens used for a

, and other properties.

While in principle a simple convex lens will suffice, in practice a compound lens made up of a number of optical lens elements is required to correct (as much as possible) the many optical aberrations that arise. Some aberrations will be present in any lens system. It is the job of the lens designer to balance these and produce a design that is suitable for photographic use and possibly mass production.

Typical rectilinear lenses can be thought of as "improved" pinhole "lenses". As shown, a pinhole "lens" is simply a small aperture that blocks most rays of light, ideally selecting one ray to the object for each point on the image sensor. Pinhole lenses have a few severe limitations:

• A pinhole camera with a large aperture is blurry because each pixel is essentially the shadow of the aperture stop, so its size is no smaller than the size of the aperture (third image). Here a pixel is the area of the detector exposed to light from a point on the object.
• Making the pinhole smaller improves resolution (up to a limit), but reduces the amount of light captured.
• At a certain point, shrinking the hole does not improve the resolution because of the
  diffraction limit
  . Beyond this limit, making the hole smaller makes the image blurrier as well as darker.

Practical lenses can be thought of as an answer to the question: "how can a pinhole lens be modified to admit more light and give a smaller spot size?". A first step is to put a simple convex lens at the pinhole with a focal length equal to the distance to the film plane (assuming the camera will take pictures of distant objects^[1]). This allows the pinhole to be opened up significantly (fourth image) because a thin convex lens bends light rays in proportion to their distance to the axis of the lens, with rays striking the center of the lens passing straight through. The geometry is almost the same as with a simple pinhole lens, but rather than being illuminated by single rays of light, each image point is illuminated by a focused "pencil" of light rays.

From the front of the camera, the small hole (the aperture), would be seen. The

are collected at the entrance pupil and focused down from the exit pupil onto the image plane.

Canon Elph

A camera lens may be made from a number of elements: from one, as in the

's meniscus lens, to over 20 in the more complex zooms. These elements may themselves comprise a group of lenses cemented together.

The front element is critical to the performance of the whole assembly. In all modern lenses the surface is coated to reduce abrasion,

are equal. In a prime lens this is easy, but in a zoom there is always a compromise.

The lens usually is focused by adjusting the distance from the lens assembly to the image plane, or by moving elements of the lens assembly. To improve performance, some lenses have a cam system that adjusts the distance between the groups as the lens is focused. Manufacturers call this different things: Nikon calls it CRC (close range correction); Canon calls it a floating system; and Hasselblad and Mamiya call it FLE (floating lens element).^[2]

Glass is the most common material used to construct lens elements, due to its good optical properties and resistance to scratching. Other materials are also used, such as

]

The 1951 USAF resolution test chart is one way to measure the resolving power of a lens. The quality of the material, coatings, and build affect the resolution. Lens resolution is ultimately limited by diffraction, and very few photographic lenses approach this resolution. Ones that do are called "diffraction limited" and are usually extremely expensive.^[7]

Today, most lenses are multi-coated in order to minimize lens flare and other unwanted effects. Some lenses have a UV coating to keep out the ultraviolet light that could taint color. Most modern optical cements for bonding glass elements also block UV light, negating the need for a UV filter. However, this leaves an avenue for lens fungus to attack if lenses are not cared for appropriately. UV photographers must go to great lengths to find lenses with no cement or coatings.

A lens will most often have an aperture adjustment mechanism, usually an iris diaphragm, to regulate the amount of light that passes. In early camera models a rotating plate or slider with different sized holes was used. These Waterhouse stops may still be found on modern, specialized lenses. A shutter, to regulate the time during which light may pass, may be incorporated within the lens assembly (for better quality imagery), within the camera, or even, rarely, in front of the lens. Some cameras with leaf shutters in the lens omit the aperture, and the shutter does double duty.

Apertures

in 1-stop increments on the same lens.

The two fundamental parameters of an optical lens are the

The maximum usable aperture of a lens is specified as the focal ratio or f-number, defined as the lens's focal length divided by the effective aperture (or entrance pupil), a dimensionless number. The lower the f-number, the higher light intensity at the focal plane. Larger apertures (smaller f-numbers) provide a much shallower depth of field than smaller apertures, other conditions being equal. Practical lens assemblies may also contain mechanisms to deal with measuring light, secondary apertures for flare reduction,^[9] and mechanisms to hold the aperture open until the instant of exposure to allow SLR cameras to focus with a brighter image with shallower depth of field, theoretically allowing better focus accuracy.

Focal lengths are usually specified in millimetres (mm), but older lenses might be marked in centimetres (cm) or inches. For a given film or sensor size, specified by the length of the diagonal, a lens may be classified as a:

• Normal lens: angle of view of the diagonal about 50° and a focal length approximately equal to the image diagonal.
• Wide-angle lens: angle of view wider than 60° and focal length shorter than normal.
• Long-focus lens: any lens with a focal length longer than the diagonal measure of the film or sensor.^[10] Angle of view is narrower. The most common type of long-focus lens is the telephoto lens, a design that uses special optical configurations to make the lens shorter than its focal length.

A side effect of using lenses of different focal lengths is the different distances from which a subject can be framed, resulting in a different

. Photographs can be taken of a person stretching out a hand with a wideangle, a normal lens, and a telephoto, which contain exactly the same image size by changing the distance from the subject. But the perspective will be different. With the wideangle, the hands will be exaggeratedly large relative to the head. As the focal length increases, the emphasis on the outstretched hand decreases. However, if pictures are taken from the same distance, and enlarged and cropped to contain the same view, the pictures will have identical perspective. A moderate long-focus (telephoto) lens is often recommended for portraiture because the perspective corresponding to the longer shooting distance is considered to look more flattering.

The widest aperture lens in history of photography is believed to be the Carl Zeiss Planar 50mm f/0.7,^[11] which was designed and made specifically for the NASA Apollo lunar program to capture the far side of the Moon in 1966. Three of these lenses were purchased by filmmaker Stanley Kubrick in order to film scenes in his 1975 film Barry Lyndon, using candlelight as the sole light source.^[12][13][14]

The complexity of a lens — the number of elements and their degree of asphericity — depends upon the angle of view, the maximum aperture, and intended price point, among other variables. An extreme wideangle lens of large aperture must be of very complex construction to correct for optical aberrations, which are worse at the edge of the field and when the edge of a large lens is used for image-forming. A long-focus lens of small aperture can be of very simple construction to attain comparable image quality: a doublet (two elements) will often suffice. Some older cameras were fitted with convertible lenses (German: Satzobjektiv) of normal focal length. The front element could be unscrewed, leaving a lens of twice the focal length, and half the angle of view and half the aperture. The simpler half-lens was of adequate quality for the narrow angle of view and small relative aperture. This would require the bellows had to be extended to twice the normal length.

Good-quality lenses with maximum aperture no greater than f/2.8 and fixed, normal, focal length need at least three (triplet) or four elements (the trade name "

Lens mounts

Many

. The lenses attach to the camera using a lens mount, which contains mechanical linkages and often also electrical contacts between the lens and camera body.

The lens mount design is an important issue for compatibility between cameras and lenses. There is no universal standard for lens mounts, and each major camera maker typically uses its own proprietary design, incompatible with other makers.

Most large-format cameras take interchangeable lenses as well, which are usually mounted in a lensboard or on the front standard.

The most common interchangeable lens mounts on the market today include the Canon

E

digital-only mount.

Types of lenses

"Close-up" or macro

A macro lens used in

, but a 1:1 ratio is, typically, considered "true" macro. Magnification from life size to larger is called "Micro" photography (2:1, 3:1 etc.). This configuration is generally used to image close-up very small subjects. A macro lens may be of any focal length, the actual focus length being determined by its practical use, considering magnification, the required ratio, access to the subject, and illumination considerations. It can be a special lens corrected optically for close up work or it can be any lens modified (with adapters or spacers, which are also known as "extension tubes".) to bring the focal plane "forward" for very close photography. Depending on the camera to subject distance and aperture, the depth-of-field can be very narrow, limiting the linear depth of the area that will be in focus. Lenses are usually stopped down to give a greater depth-of-field.

Zoom

Some lenses, called zoom lenses, have a focal length that varies as internal elements are moved, typically by rotating the barrel or pressing a button which activates an electric motor. Commonly, the lens may zoom from moderate wide-angle, through normal, to moderate telephoto; or from normal to extreme telephoto. The zoom range is limited by manufacturing constraints; the ideal of a lens of large maximum aperture which will zoom from extreme wideangle to extreme telephoto is not attainable. Zoom lenses are widely used for small-format cameras of all types: still and cine cameras with fixed or interchangeable lenses. Bulk and price limit their use for larger film sizes. Motorized zoom lenses may also have the focus, iris, and other functions motorized.

• Apochromat (apo) lenses have added correction for chromatic aberration.
• barrel distortion
  ) and are generally intended for use at a specific distance and at small aperture.
• Enlarger lenses are made to be used with
  photographic enlargers
  (specialised projectors), rather than cameras.
• Lenses for aerial photography.
• Shift lens allow the lens to be raised or lowered relative to the film of sensor plane to correct or exaggerate perspective distortion.
• Fisheye lenses: extreme wide-angle lenses with an angle of view of up to 180 degrees or more, with very noticeable (and intended) distortion.
• Stereoscopic lenses, to produce pairs of photographs which give a 3-dimensional effect when viewed with an appropriate viewer.
• Soft-focus lenses which give a soft, but not out-of-focus, image and have an imperfection-removing effect popular among portrait and fashion photographers.
• Infrared lenses
• Ultraviolet lenses
• Swivel lenses rotate while attached to a camera body to give unique perspectives and camera angles.
• Shift lenses and tilt/shift lenses (collectively
  SLR cameras by mimicking view camera movements
  .
• telecentric lenses (or orthographic lenses) make any object appear as the same size regardless of their distance from the lens.

Some notable photographic optical lens designs are:

• Angenieux retrofocus
• Cooke triplet
• Double-Gauss
• Goerz Dagor
• Leitz Elmar
• Rapid Rectilinear
• Zeiss Sonnar
• Zeiss Planar
• Zeiss Tessar

• Anti-fogging treatment of optical surfaces
• Large format lens
• Lens (optics)
• Lens hood
• Lens cover
• Lenses for SLR and DSLR cameras
• Optical train
• Teleconverter
• Teleside converter
• William Taylor (inventor)

Sources

• ISBN 978-0-12-408640-1
  .
• Guy, N. K. (2012). The Lens: A Practical Guide for the Creative Photographer. Rocky Nook.
  ISBN 978-1-933952-97-0
  .

External links

Wikimedia Commons has media related to Photographic lenses.

• Photo.net Lens Tutorial
• optical glass