A camera lens (also known as photographic lens or photographic objective) 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.
There is no major difference in principle between a lens used for a still camera, a video camera, a telescope, a microscope,
or other apparatus, but the detailed design and construction are
different. A lens may be permanently fixed to a camera, or it may be interchangeable with lenses of different focal lengths, apertures, 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 out and produce a design
that is suitable for photographic use and possibly mass production.
Theory of operation
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 (below left). 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.
- Diffraction limits the effectiveness of shrinking the hole, so at a point, making the hole smaller makes the image blurrier as well as darker (below center).
Practical lenses can be thought of as an answer to the question "how
can we modify a pinhole lens 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 us to open up the pinhole significantly (below right)
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.
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Standing in front of the camera, you would see the small hole, the aperture. The virtual image of the aperture as seen from the world is known as the lens's entrance pupil;
ideally, all rays of light leaving a point on the object that enter the
entrance pupil will be focused to the same point on the image
sensor/film (provided the object point is in the field of view). If one
were inside the camera, one would see the lens acting as a projector. The virtual image of the aperture from inside the camera is the lens's exit pupil.
In this simple case, the aperture, entrance pupil, and exit pupil are
all in the same place because the only optical element is in the plane
of the aperture, but in general these three will be in different places.
Practical photographic lenses include more lens elements. The
additional elements allow lens designers to reduce various aberrations,
but the principle of operation remains the same: pencils of rays are collected at the entrance pupil and focused down from the exit pupil onto the image plane.
Construction
Main articles: Photographic lens design and History of photographic lens design
A camera lens may be made from a number of elements: from one, as in the Box Brownie'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,
flare, and surface reflectance, and to adjust color balance. To minimize aberration, the curvature is usually set so that the angle of incidence and the angle of refraction 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 quartz glass, fluorite,[3][4][5][6] plastics like acrylic (Plexiglass), and even germanium and meteoritic glass.[7] Plastics allow the manufacturing of strongly aspherical lens
elements which are difficult or impossible to manufacture in glass, and
which simplify or improve lens manufacturing and performance.[citation needed]
Plastics are not used for the outermost elements of all but the
cheapest lenses as they scratch easily. Molded plastic lenses have been
used for the cheapest disposable cameras for many years, and have
acquired a bad reputation: manufacturers of quality optics tend to use
euphemisms such as "optical resin". However many modern, high
performance (and high priced) lenses from popular manufacturers include
molded or hybrid aspherical elements, so it is not true that all lenses
with plastic elements are of low photographic quality.
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.[8]
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.
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.
Aperture and focal length
The two fundamental parameters of an optical lens are the focal length and the maximum aperture.
The lens' focal length determines the magnification of the image
projected onto the image plane, and the aperture the light intensity of
that image. For a given photographic system the focal length determines
the angle of view,
short focal lengths giving a wider field of view than longer focal
length lenses. A wider aperture, identified by a smaller f-number,
allows using a faster shutter speed for the same exposure.[9]
The maximum usable aperture of a lens is specified as the focal ratio or f-number, defined as the lens' 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,[10] 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.[11] 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 perspective.
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,[12] 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 movie Barry Lyndon, using candlelight as the sole light source.[13][14][15]
| An example of how lens choice affects angle of view. The photos were taken by a 35 mm camera at a constant distance from the subject. | |||||||||
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Number of elements
Main article: Photographic lens design
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 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. Obviously the bellows had to extend 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 "Tessar" derives from the Greek tessera,
meaning "four"). The widest-range zooms often have fifteen or more. The
reflection of light at each of the many interfaces between different
optical media (air, glass, plastic) seriously degraded the contrast and color saturation
of early lenses, particularly zoom lenses, especially where the lens
was directly illuminated by a light source. The introduction many years
ago of optical coatings,
and advances in coating technology over the years, have resulted in
major improvements, and modern high-quality zoom lenses give images of
quite acceptable contrast, although zoom lenses with many elements will
transmit less light than lenses made with fewer elements (all other
factors such as aperture, focal length, and coatings being equal).[16]
Lens mounts
Main article: Lens mount
Many Single-lens reflex cameras, and some rangefinder cameras have detachable lenses. A few other types do as well, notably the Mamiya TLR cameras and mirrorless interchangeable-lens cameras. 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.[17] A few older manual focus lens mount designs, such as the Leica M39 lens mount for rangefinders, M42 lens mount for early SLRs, and the Pentax K mount are found across multiple brands, but this is not common today. A few mount designs, such as the Olympus/Kodak Four Thirds System mount for DSLRs, have also been licensed to other makers.[18]
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 EF, EF-S and EF-M autofocus lens mounts, the Nikon F manual and autofocus mounts, the Olympus/Kodak Four Thirds and Olympus/Panasonic Micro Four Thirds digital-only mounts, the Pentax K mount and autofocus variants, the Sony Alpha mount (derived from the Minolta mount) and the Sony E digital-only mount.
Types of lens
"Close-up" or macro
A macro lens used in macro or "close-up" photography (not to be confused with the compositional term close up)
is any lens that produces an image on the focal plane (i.e., film or a
digital sensor) that is the same size or larger than the subject being
imaged. 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. They can be special lens corrected
optically for close up work or they can be any lens modified (with
adapters or spacers) to bring the focal plane "forward" for very close
photography. The depth-of-field is very narrow, limiting their
usefulness. Lenses are usually stopped down to give a greater
depth-of-field.[9][19]
Zoom
Main article: Zoom lens
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.
Special-purpose
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