For general lens design see Lens design
The design of
photographic lensA photographic 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,...
es (or
photographic lens design) for use in still or
cineÇine is a town and a district of Aydın Province, in the Aegean region of Turkey, from the city of Aydın, on the road to Muğla.- Geography :...
camerathumb |right|Cameras from Large to Small, Film to Digital A camera is a device that records images, either as a still photograph or as moving images known as videos or movies...
s is intended to produce a lens that yields the most acceptable rendition of the subject being
photographedPhotography is the process, activity and art of creating still or moving pictures by recording radiation on a sensitive medium, such as a photographic film, or an electronic sensor...
within a range of constraints that include cost, weight and materials. For many other optical devices such as
telescopeA telescope is an instrument designed for the observation of remote objects by the collection of electromagnetic radiation. The first known practically functioning telescopes were invented in the Netherlands at the beginning of the 17th century...
s,
microscopeA microscope is an instrument to see objects too tiny for the naked eye. The science of investigating small objects using such an instrument is called microscopy. Microscopic means invisible to the eye unless aided by a microscope.-History:An early microscope was made in 1590 in Middelburg, The...
s and
theodoliteA theodolite is an instrument for measuring both horizontal and vertical angles, as used in triangulation networks. It is a key tool in surveying and engineering work, particularly on inaccessible ground, but theodolites have been adapted for other specialized purposes in fields like meteorology...
where the visual image is observed but often not recorded the design can often be significantly simpler than is the case in a camera where every image is captured on
filmFilm encompasses individual motion pictures, the field of film as an art form, and the motion picture industry. Films are produced by recording images from the world with cameras, or by creating images using animation techniques or visual effects....
or
image sensorAn image sensor is a device that converts an optical image to an electric signal. It is used mostly in digital cameras and other imaging devices. An image sensor is typically a charge-coupled device or a complementary metal–oxide–semiconductor active-pixel sensor.- CCD vs CMOS :Today, most...
and can be subject to detailed scrutiny at a later stage.
Design requirements
From the perspective of the photographer, the ability of a
lensA lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens is a lens consisting of a single optical element...
to capture sufficient light so that the camera can operate over a wide range of lighting conditions is important. Designing a lens that reproduces colour accurately is also important as is the production of an evenly lit and sharp image over the whole of the film or sensor plane.
For the lens designer, achieving these objectives will also involve ensuring that internal
flareLens flare is the light scattered in lens systems through generally unwanted image formation mechanisms, such as internal reflection and scattering from material inhomogeneities in the lens. These mechanisms differ from the intended image formation mechanism that depends on refraction of the image...
, optical aberrations and weight are all reduced to the minimum whilst
zoomA zoom lens is a mechanical assembly of lens elements with the ability to vary its focal length , as opposed to a fixed focal length lens...
,
focusIn 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...
and
apertureIn 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,...
functions all operate smoothly and predictably.
However, because photographic films and electronic sensors have a finite and measurable resolution, photographic lenses are not always designed for maximum possible resolution since the recording medium would not be able to record the level of detail that the lens could resolve. For this, and many other reasons, camera lenses are unsuited to use as
projectorA slide projector is an opto-mechanical device to view photographic slides. It has four main elements: a fan-cooled electric incandescent light bulb or other light source, a reflector and "condensing" lens to direct the light to the slide, a holder for the slide and a focusing lens...
or
enlargerAn enlarger is a specialized transparency projector used to produce photographic prints from film or glass negatives using the gelatin-silver process, or transparencies. All enlargers consist of a light source - normally an incandescent light bulb, a holder for the negative or transparency and a...
lenses.
The design of a fixed focal length lens (also erroneously known as
prime lensIn film and photography, a prime lens is either a photographic lens whose focal length is fixed, as opposed to a zoom lens, or it is the primary lens in a combination lens system....
es) presents fewer challenges than the design of a zoom lens. A high quality prime lens whose focal length is about equal to the diameter of the film frame or sensor may be constructed from as few as four separate lens elements, often as matched pairs on either side of the aperture diaphragm. Good examples includes the Zeiss
TessarThe Tessar is a famous photographic lens design conceived by physicist Paul Rudolph in 1902 while he worked at the Zeiss optical company and patented by Zeiss; the lens type is usually known as the Zeiss Tessar....
or the Leitz Elmar.
Design constraints
To be useful in photography any lens must be able to fit the camera for which it is intended and this will physically limit the size where the
bayonet mountA bayonet mount or bayonet connector is a fastening mechanism consisting of a male side with one or more pins, and a female receptor with matching "L" slots and spring to keep the two parts locked together....
ing or
screw mountingA lens mount is an interface — mechanical and often also electrical — between a photographic camera body and a lens. It is confined to cameras where the body allows interchangeable lenses, most usually the single lens reflex type or any movie camera of 16 mm or higher gauge...
is to be located.
Photography is a highly competitive commercial business and both weight and cost constrain the production of lenses.
Refractive materials such as glass have physical limitations which limit the performance of lenses. In particular the range of refractive indices available in commercial glasses span a very narrow range. Since it is the
refractive indexThe refractive index of a medium is a measure of how much the speed of light is reduced inside the medium. For example, typical soda-lime glass has a refractive index close to 1.5, which means that in glass, light travels at 1 / 1.5 = 2/3 the speed of light in a vacuum...
that determines how much the rays of light are bent at each interface and since it is the differences in refractive indices in paired plus and minus lenses that constrains the ability to minimise
chromatic aberrationIn optics, chromatic aberration is the failure of a lens to focus all colors to the same point. It occurs because lenses have a different refractive index for different wavelengths of light...
s, having only a narrow spectrum of indices is a major design constraint.
History
The lenses of the very earliest cameras were simple meniscus or simple bi convex lenses. It was not until 1840 that Chevalier in France introduced the achromatic lens formed by cementing a
crown glassCrown glass is type of optical glass used in lenses and other optical components.Crown glass is produced from alkali-lime silicates containing approximately 10% potassium oxide...
bi-convex lens to a
flint glassFlint glass is optical glass that has relatively high refractive index and low Abbe number. Flint glasses are arbitrarily defined as having an Abbe number of 50 to 55 or less. The currently known flint glasses have refractive indices ranging between 1.45 and 2.00...
plano-concave lens.
By 1841
VoigtländerVoigtländer is an optical company founded by Johann Christoph Voigtländer in Vienna in 1756 and thus the oldest name in cameras. It produced the Petzval photographic lens in 1840, and the world's first all-metal daguerrotype camera in 1841, also bringing out plate cameras shortly afterwards...
working with Joseph Petzval in
AustriaAustria , officially the Republic of Austria , is a landlocked country of roughly 8.3 million people in Central Europe. It borders both Germany and the Czech Republic to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the west...
developed the first true two element lens.
The role of Zeiss
The Zeiss company was responsible for many innovations in optical design and engineering. Early on, Carl Zeiss realised that he needed a competent designer so as to take the firm beyond just being another optical workshop. In 1866, the service of Dr Ernst Abbe was enlisted. From then on novel products appeared in rapid succession which brought the Zeiss company to the forefront of optical technology.
Abbe was instrumental in the development of the famous Jena optical glass. When he was trying to eliminate astigmatism from microscopes, he realised that the range of optical glasses available was insufficient. After some calculations, he realised that performance of optical instruments would dramatically improve, if optical glasses of appropriate properties were available. His challenge to glass manufacturers was finally answered by Dr Otto Schott, who established the famous glassworks at
JenaJena is a university city in central Germany on the river Saale. With a population of 103,000 it is the second largest city in the federal state of Thuringia, after Erfurt.-History:Jena was first mentioned in an 1182 document...
from which new types of optical glass began to appear from 1888, and employed by Zeiss and other makers.
The new Jena optical glass also opened up the possibility of increased performance of photographic lenses. The first use of Jena glass in a photographic lens was by
VoigtländerVoigtländer is an optical company founded by Johann Christoph Voigtländer in Vienna in 1756 and thus the oldest name in cameras. It produced the Petzval photographic lens in 1840, and the world's first all-metal daguerrotype camera in 1841, also bringing out plate cameras shortly afterwards...
, but as the lens was an old design its performance was not greatly improved. Subsequently the new glasses would demonstrate their value in correcting
astigmatismAn optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances...
, and in the production of apochromatic lenses. Abbé started the design of a photographic lens of symmetrical design with five elements, but went no further.
Zeiss' domination of photographic lens innovation was due to Dr
Paul RudolphPaul Rudolph was a German physicist who designed the first anastigmatic lens while working for Carl Zeiss. After World War I, he joined the Hugo Meyer optical company, where he designed most of their cine lenses. -Work:...
. In 1890, Rudolph designed an asymmetrical lens with a cemented group at each side of the diaphragm, and appropriately named "Anastigmat". This lens was made in three series: Series III, IV and V, with maximum apertures of f/7.2, f/12.5, and f/18 respectively. In 1891, Series I, II and IIIa appeared with respective maximum apertures of f/4.5, f/6.3, and f/9 and in 1893 came Series IIa of f/8 maximum aperture. These lenses are now better known by the trademark "Protar" which was first used in 1900.
At the time, single combination lenses, which occupy one side of the diaphragm only, were still popular. Rudolph designed one with three cemented elements in 1893, with the option of fitting two of them together in a lens barrel as a compound lens, but it was found to be the same as the Dagor by C.P. Goerz, designed by Emil von Hoegh. Rudolph then came up with a single combination with four cemented elements, which can be considered as having all the elements of the Protar stuck together in one piece. Marketed in 1894, it was called the Protarlinse Series VII, the most highly corrected single combination lens with maximum apertures between f/11 and f/12.5, depending on its focal length.
But the important thing about this Protarlinse is that two of these lens units can be mounted in the same lens barrel to form a compound lens of even greater performance and larger aperture, between f/6.3 and f/7.7. In this configuration it was called the Double Protar Series VIIa. An immense range of focal lengths can thus be obtained by the various combination of Protarlinse units.
Rudolph also investigated the
Double-GaussThe double Gauss lens consists of two Gauss lenses back to back.In its most basic design it therefore consists of two positive meniscus lenses on the outside with two negative meniscus lenses inside them. The symmetry of the system and the splitting of the optical power into many elements reduces...
concept of a symmetrical design with thin positive meniscii enclosing negative elements. The result was the Planar Series Ia of 1896, with maximum apertures up to f/3.5, one of the fastest lenses of its time. Whilst it was very sharp, it suffered from
comaIn optics , the coma in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components which results in off-axis point sources such as stars appearing distorted. Specifically, coma is defined as a variation in magnification over...
which limited its popularity. However, further developments of this configuration made it the design of choice for high-speed lenses of standard coverage.
Probably inspired by the Stigmatic lenses designed by Hugh Aldis for Dallmeyer of London, Rudolph designed a new asymmetrical lens with four thin elements, the Unar Series Ib, with apertures up to f/4.5. Due to its high speed it was used extensively on hand cameras.
The most important Zeiss lens by Rudolph was the
TessarThe Tessar is a famous photographic lens design conceived by physicist Paul Rudolph in 1902 while he worked at the Zeiss optical company and patented by Zeiss; the lens type is usually known as the Zeiss Tessar....
, first sold in 1902 in its Series IIb f/6.3 form. It can be said as a combination of the front half of the Unar with the rear half of the Protar. This proved to be a most valuable and flexible design, with tremendous development potential. Its maximum aperture was increased to f/4.7 in 1917, and reached f/2.7 in 1930. It is probable that every lens manufacturer has produced lenses of the Tessar configuration.
Rudolph left Zeiss after the First World War, but many other competent designers such as Merté, Wandersleb, etc. kept the firm at the leading edge of photographic lens innovations. One of the most significant designer was the ex-Ernemann man Dr Ludwig Bertele, famed for his Ernostar high-speed lens.
With the advent of the
ContaxContax was a camera brand noted for its unique, and sometimes odd, technical innovation and a wide range of Zeiss lenses, noted for their high optical quality...
by Zeiss-Ikon, the first serious challenge to the Leica in the field of professional 35 mm cameras, both Zeiss-Ikon and Carl Zeiss decided to beat the Leica in every possible way. Bertele's Sonnar series of lenses designed for the Contax were the match in every respect for the Leica for at least two decades. Other lenses for the Contax included the Biotar, Biogon, Orthometar, and various Tessars and Triotars.
The last important Zeiss innovation before the Second World War was the technique of applying anti-reflective coating to lens surfaces. A lens so treated was marked with a red "T", short for "Transparent". The technique of applying multiple layers of coating was developed from this basis after the war, and known as "T*" (T-star).
After the partitioning of Germany, a new Carl Zeiss optical company was established in
OberkochenOberkochen is a town in the Ostalbkreis, in Baden-Württemberg, Germany.- Economy :After WWII the allied troops moved parts of the Carl Zeiss Company in Jena to Oberkochen. Today the headquarter of the Carl Zeiss AG is located in Oberkochen. Another important business is the Leitz company, which is...
, while the original Zeiss firm in
JenaJena is a university city in central Germany on the river Saale. With a population of 103,000 it is the second largest city in the federal state of Thuringia, after Erfurt.-History:Jena was first mentioned in an 1182 document...
continued to operate. At first both firms produced very similar lines of products, and extensively cooperated in product-sharing, but they drifted apart as time progressed. Jena's new direction was to concentrate on developing lenses for the 35 mm single-lens reflex camera, and many achievements were made, especially in ultra-wide angle designs. In addition to that, Oberkochen also worked on designing lenses for large format cameras, interchangeable front element lenses such as for the 35 mm single-lens reflex Contaflex, and other types of cameras.
Since the beginning of Zeiss as a photographic lens manufacturer, it has had a licensing programme which allows other manufacturers to produce its lenses. Over the years its licensees included
VoigtländerVoigtländer is an optical company founded by Johann Christoph Voigtländer in Vienna in 1756 and thus the oldest name in cameras. It produced the Petzval photographic lens in 1840, and the world's first all-metal daguerrotype camera in 1841, also bringing out plate cameras shortly afterwards...
,
Bausch & LombBausch & Lomb is an American company based in Rochester, New York, is one of the world's leading suppliers of eye health products, such as contact lenses and lens care products today. In addition to this main activity, in recent years the area of medical technology has been developed...
, Ross, Koristka, Krauss, Kodak. etc. In the 1970s, the western operation of Zeiss-Ikon got together with Yashica to produce the new
ContaxContax was a camera brand noted for its unique, and sometimes odd, technical innovation and a wide range of Zeiss lenses, noted for their high optical quality...
cameras, and many of the Zeiss lenses for this camera, among others, were produced by Yashica's optical arm, Tomioka. As Yashica's owner
Kyocerais a multinational manufacturer based in Kyoto, Japan. It was founded as in 1959 by Kazuo Inamori and renamed in 1982. The company has diversified its founding technology in ceramic materials through internal development as well as strategic mergers and acquisitions...
ended camera production in 2006, and Yashica lenses were then made by
Cosinais a designer and manufacturer of cameras and lenses, and a glassmaker, based in Nakano, Nagano Prefecture, Japan.Cosina is the successor to Nikō , a company set up as a manufacturer oflenses in 1959...
, who also manufactured most of the new Zeiss designs for the new Zeiss Ikon coupled rangefinder camera. Another licensees active today is
Sonyis a multinational conglomerate corporation headquartered in Minato, Tokyo, Japan, and one of the world's largest media conglomerates with revenue exceeding ¥ 7.730.0 trillion, or $78.88 billion U.S. . Sony is one of the leading manufacturers of electronics, video, communications, video game...
who uses the Zeiss name on lenses on its video and digital still cameras
Lens elements
Except for the most simple and inexpensive lenses, each complete lens is made up from a number of separate lens elements arranged along a common axis. The use of many lens elements is designed to minimise aberrations and to provide a sharp image, free from flare and other optical imperfections. To do this requires lens elements of different compositions and different shapes. To minimise chromatic aberrations, in which different wavelengths of light are refracted to different degrees, requires, at a minimum, a couplet of lens elements with a positive element having a high refractive index matched with a negative element of lower refractive index. If the front element is a
meniscusMeniscus, plural: menisci/meniscuses, from the Greek for "crescent", is a curve in the surface of a molecular substance and is produced in response to the surface of the container or another object. It can be either concave or convex. A convex meniscus occurs when the molecules have a stronger...
design and the second is a bi-concave design, it is relatively simple to achieve a good degree of convergence of different wavelengths in the
visible spectrumThe visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 380 to 750 nm...
. Most lens designs do not attempt to bring
infraredInfrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves...
wavelengths to the same common focus and it is therefore necessary to manually alter the focus when photographing in infrared light.
Most lens elements are made with curved surfaces with a
sphericalA sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
profile. That is the curved shape would fit on the surface of a sphere. This is partly to do with the history of lens making but also because grinding and manufacturing of spherical surface lenses is relatively simple and cheap. However, spherical surfaces also give rise to lens aberrations and can lead to heavy lens designs. Achieving higher quality lenses and lower weights can be achieved by using
aspheric lensAn aspheric lens or asphere is a lens whose surfaces have a profile that is neither a portion of a sphere nor of a circular cylinder. In photography, a lens assembly that includes an aspheric element is often called an aspherical lens....
es in which the curved surfaces are not spherical.
Types of lenses
The type of lens being designed is significant in setting the key parameters.
- Prime lens
In film and photography, a prime lens is either a photographic lens whose focal length is fixed, as opposed to a zoom lens, or it is the primary lens in a combination lens system....
- usually the simplest to design. In very inexpensive cameras this can be a simple meniscus lens. Such a simple lens exhibits a number of failings including circular and colour aberrations. In addition it produces a curved field of illumination. In the plastic bodied Kodak Box Brownie this problem was minimised by using a curved track for the film.
- Wide angle lens - the problem posed by the design of wide angle lenses is to bring an accurate focus light from a wide area without causing internal flare. Wide angle lenses therefore tend to have more elements than a prime lens to help refract the light sufficiently and still minimise aberrations whilst adding light-trapping baffles between each lens element.
- Extreme wide angle lenses share the same issues as ordinary wide angle lenses but the focal length of such lenses may be so short that there is insufficient physical space in front of the film or sensor plane to construct a lens. This problem is resolved by constructing the lens as an inverted telephoto, or retrofocus
The Angénieux retrofocus photographic lens is a wide-angle lens design that uses an inverted telephoto configuration. The popularity of this lens design made the name retrofocus synonymous with this type of lens...
, with the front element having a very short focal length, often with a highly exaggerated convex front surface and behind it a strongly negative lens grouping that extends the cone of focused rays so that they can be brought to focus at a reasonable distance.
- Long focus lenses, in which the focal length is significantly greater than the diagonal of the film frame or sensor, are relatively simple to design, the challenges being comparable to the design of a prime lens. However, as the focal length increases the length of the lens and the size of the objective increase in size and length and weight quickly become significant design issues in retaining utility and practicality for the lens in use. In addition because the light path through the lens is long and glancing, the importance of baffles to control flare increases in importance.
- Telephoto lens
In photography and cinematography, a telephoto lens is a specific construction of a long focal length photographic lens in which the physical length of the lens is shorter than the focal length. In these lenses the optical centre lies outside of its physical construction, such that the entire lens...
- the design of telephoto lenses reduces some of the problems encountered by designers of long focus lenses but introduces others.
- Zoom lens
A zoom lens is a mechanical assembly of lens elements with the ability to vary its focal length , as opposed to a fixed focal length lens...
es - cover a range of focal lengths by utilising movable elements within the barrel of the lens assembly. In early zoom lenses the focus also shifted as the lens focal length was changed. All modern zoom lenses are now confocal, meaning that the focus is maintained throughout the zoom range. Because of the need to operate over a range of focal lengths and maintain confocality, zoom lenses typically have very many lens elements. More significantly the front elements of the lens will always be a compromise in terms of its size, light-gathering capability and the angle of incidence of the incoming rays of light. For all these reasons, the optical performance of zoom lenses tends to be lower than fixed focal length lenses.
- Mirror lens - Mirror lenses are a form of telephoto lens but with a light path that doubles back on itself and with an objective that is a Catadioptric mirror rather than a lens. A second centrally-placed mirror and small lens group bring the light to focus. Such lenses are very lightweight and can easily deliver very long focal lengths but they can only deliver a fixed aperture and have none of the benefits of being able to stop down the aperture to increase depth of field.
Specialist lenses
Anamorphic lenses are used principally in
cinematographyCinematography , is the making of lighting and camera choices when recording photographic images for the cinema. It is closely related to the art of still photography...
to produce wide-screen films where the projected image has a substantially different ratio of height to width than the image recorded on the film plane. This is achieved by the use of a specialised lens design which compresses the image laterally at the recording stage and the film is then projected through a similar lens in the cinema to recreate the wide-screen effect. Although in some cases the anamorphic effect is achieved by using an anamorphising attachment as a supplementary element on the front of a normal lens, most films shot in anamorphic formats use specially-designed anamorphic lenses, such as the Hawk lenses made by Vantage Film or
Panavision'sPanavision is a motion picture equipment company specializing in cameras and lenses, based in Woodland Hills, California. Formed by Robert Gottschalk as a small partnership to create anamorphic projection lenses during the widescreen boom in the 1950s, Panavision expanded its product lines to meet...
anamorphic lenses. These lenses incorporate one or more
aspheric elementsAn aspheric lens or asphere is a lens whose surfaces have a profile that is neither a portion of a sphere nor of a circular cylinder. In photography, a lens assembly that includes an aspheric element is often called an aspherical lens....
in their design.
Lens glass
The majority of photographic lenses have the lens elements made from glass although the use of high quality plastics is becoming more common in high quality lenses and has been common in more inexpensive cameras for some time. The design of photographic lenses is very demanding as designers push the limits of existing materials to make more versatile, better quality and lighter lenses. As a consequence many exotic glasses have been used in modern lens manufacturers.
CaesiumCaesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of , which makes it one of only five metals that are liquid at or near room temperature...
and
lanthanumLanthanum is a chemical element with the symbol La and atomic number 57.Lanthanum is a silvery white metallic element that belongs to group 3 of the periodic table and is a lanthanoid. It is found in some rare-earth minerals, usually in combination with cerium and other rare earth elements...
glass lenses are now in use because of their high refractive index and very low dispersion properties. It is also likely that a number of other transition element glasses are in use but manufacturers often prefer to keep their material specification secret to retain a commercial or performance edge over their rivals.
Aperture control
The aperture control, usually a multi-leaf diaphragm, is critical to the performance of a lens. The role of the aperture is to control the amount of light passing through the lens to the film or sensor plane. An aperture placed outside of the lens, as in the case of some
VictorianThe Victorian era of the United Kingdom was the period of Queen Victoria's reign from June 1837 until her death on the 22nd of January 1901. The reign was a long period of prosperity for the British people, as profits gained from the overseas British Empire, as well as from industrial improvements...
cameras risks vignetting of the image in which the corners of the image are darker than the centre. A diaphragm too close to the image plane risks the diaphragm itself being recorded as a circular shape or at the very least causing diffraction patterns at small apertures. In most lens designs the aperture is positioned about mid-way between the front surface of the objective and the image plane. In some zoom lenses it is placed some distance away from the ideal location in order to accommodate the movement of floating lens elements needed to perform the zoom function.
Most modern lenses for 35mm format rarely provide a stop smaller than f22 because of the diffraction effects caused by light passing through a very small aperture. As diffraction is based on aperture width in absolute terms rather than the f-stop ratio, lenses for very small formats common in compact cameras rarely go above f/11 (1/1.8") or f/8 (1/2.5"), while lenses for medium- and large-format provide f/64 or f/128.
Very large aperture lenses designed to be useful in very low light conditions with apertures ranging from f1.2 to f0.9 are generally restricted to lenses of standard focal length because of the of the size and weight problems that would be encountered in telephoto lenses and the difficulty of building a very wide aperture wide angle lens with the refractive materials currently available. Very large aperture lenses are commonly made for other types of optical instruments such as microscopes but in such cases the diameter of the lens is very small and weight is not an issue.
Many very early cameras had diaphragms external to the lens often consisting of a rotating circular plate with a number of holes of increasing size drilled through the plate. Rotating the plate would bring an appropriate sized hole in front of the lens. All modern lenses use a multi-leaf diaphragm so that at the central intersection of the leaves a more or less circular aperture is formed; either a manual ring control or an electronic motor controls the angle of the diaphragm leaves and thus the size of the opening.
The placement of the diaphragm within the lens structure is constrained by the need to achieve even illumination over the whole film plane at all apertures and the requirement to not interfere with the movement of any movable lens element. Typically the diaphragm is situated at about the level of the optical centre of the lens.
Shutter mechanism
A
shutterIn photography, a shutter is a device that allows light to pass for a determined period of time, for the purpose of exposing photographic film or a light-sensitive electronic sensor to light to capture a permanent image of a scene...
controls the length of time light is allowed to pass through the lens onto the film plane. For any given light intensity, the more sensitive the film or detector or the wider the aperture the shorter the
exposureIn photography, exposure is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph...
time need to be to maintain the optimal exposure.
In the earliest camera exposures were controlled by moving a rotating plate from in front of the lens and then replacing it. Such a mechanism only works effectively for exposures of several seconds or more and carries a considerable risk of inducing
camera shakeCamera shake is a common problem in photography when telephoto lenses or long shutter speeds are used with a non-stabilized camera. The resulting images will appear blurry and smeared...
. By the end of the 19th century spring tensioned shutter mechanisms were in use operated by a lever or by a cable release. Some simple shutters continued to be placed in front of the lens but most were incorporated within the lens mount itself.
Such lenses with integral shutter mechanisms developed in the current Compur shutter as used in many non-reflex cameras such as
LinhofLinhof is a German company, founded in Munich in 1887 by Valentin Linhof. The company is well known for making premium rollfilm and large format film cameras. The company began by making camera shutters, and Linhof developed the leaf shutter, which became part of Compur...
. These shutters have a number of metal leaves that spring open and then close after a pre-determined interval. The material and design constraints limit the shortest speed to about 0.002 second. Although such shutters cannot yield as short and exposure time as focal plane shutter they are able to offer
flash synchronisationIn a camera, flash synchronization is defined as the firing of a photographic flash coinciding with the shutter admitting light to photographic film or electronic image sensor. It is often shortened to flash sync or flash synch....
at all speeds.
Incorporating a commercial made Compur type shutter required lens designers to accommodate the width of the shutter mechanism in the lens mount and provide for the means of triggering the shutter on the lens barrel or transferring this to the camera body by a series of levers as in the
MinoltaMinolta Co., Ltd. was a Japanese worldwide manufacturer of cameras, camera accessories, photocopiers, fax machines, and laser printers. Minolta was founded in Osaka, Japan, in 1928 as Nichi-Doku Shashinki Shōten . It is perhaps best-known for making the first integrated autofocus 35 mm SLR...
twin lens cameras.
The need to accommodate the shutter mechanism within the lens barrel limited the design of wide-angle lenses and it was not until the widespread use of focal plane shutters that con-focal zoom lenses were developed.
Focus
Until recent years focusing of a camera lens to achieve a sharp image on the film plane was achieved by means of a very shallow helical thread in the lens mount through which the lens could be rotated moving it closer or further from the film plane. This arrangement whilst simple to design and construct has some limitations not least the rotation of the greater part of the lens assembly including the front element. This could be problematical if devices such as polarising filters were in use that require to maintain an accurate vertical orientation irrespective of focus distance.
Later developments adopted designs in which internal elements were moved to achieve focus without affecting the outer barrel of the lens or the orientation of the front element.
Many modern cameras now use automatic focusing mechanisms which use
ultrasonic motorAn ultrasonic motor is a type of electric motor formedfrom the ultrasonic vibration of a component, the stator, placedagainst another, the rotor or slider depending on the schemeof operation...
s to move internal elements in the lens to achieve optimum focus.