Lens (optics)

Bright normal photo lens with maximum open aperture
Two high-resolution microscope objectives
2 historical lenses Carl Zeiss , Jena, No. 145077 and No. 145078, Tessar 1: 4.5 F = 5.5 cm DRP 142294 (built before 1910)

A lens is a collecting optical system that creates a real optical image of an object (object). It is the most important component of imaging optical devices, for example cameras , binoculars , microscopes , projectors or astronomical telescopes . The word lens is an abbreviated form of lens glass that has been attested since the 18th century. The lens glass stands between the object and the image.

properties

The simplest lens is a single converging lens , as the first telescopes had around 1608 . Components of an objective can, however, be lenses , mirrors or (less often) diffraction gratings , which, depending on the intended use, are located in one or more tubes that are blackened and ribbed on the inside to reduce stray light . The main characteristics of a lens are its focal length , which determines the image scale for a given object distance , and the aperture (free opening of the front lens).

Other important properties are the image quality, which is determined by a suitable combination of several lenses with different refractive indices, thicknesses and radii of curvature and serves to reduce optical imaging errors, as well as the scattered light sensitivity, which should be as low as possible. Scattered light sensitivity is particularly important with backlighting and can be reduced by blackened panels and coatings .

Further properties are the photographic light intensity (aperture ratio) and the close-up limit , which determines how close you can "get" to the subject (see macro lens ).

In camera systems , the mechanical, electrical and electronic connection of lenses to a camera housing is specified. The lens connection is usually implemented with an objective thread or with an objective bayonet . Electrical components of the lens, such as motors for changing the set object length, changing the focal length or image stabilization , can be supplied with energy via the electrical connections . The electronic connections can be used to exchange unidirectional or bidirectional digital information between the lens and the camera housing. There are autofocus lenses for this , in which the distance setting can be controlled or regulated with the help of electric motors . Furthermore, the focal length of lenses with adjustable focal length (see zoom lens ) can be set using motors.

Lenses from digital camera systems are usually equipped with digital technology and are able to communicate with the camera housing via digital interfaces .

Focal length and focus

The size of the image is determined by the focal length f of the lens and the distance g between the object and the lens. As an approximation, objects that are optically “at infinity” (as a rule of thumb: distances greater than 20 times the focal length) are imaged directly in the focal plane of the lens, which also contains its focal point (focus). Objects located closer are only shown a little behind the focal point, whereby this image distance b results in simplified form from the distance (object distance g ) and the lens equation

${\ displaystyle {\ frac {1} {b}} + {\ frac {1} {g}} = {\ frac {1} {f}}}$.

Just like the object, the image created is three- dimensional. However, only in a plane , the image plane as viewed or photographed, and therefore requires - depending on the distance of the object - a focus adjustment ( focusing ):

• for telescopes and binoculars by moving the eyepiece (which has the function of a magnifying glass)
• in cameras by moving lenses or optical groups in the lens
• in microscopes by moving the entire optical system (change of g ).

It is shifted either manually via a fine thread or, in the case of devices with autofocus , by small step motors . Earlier cameras had an extension ( bellows ) running on metal rods , which was sometimes used for lenses of different focal lengths. The bellows principle is still used today in large picture and macro photography .

Systematics

Lenses are primarily differentiated according to their intended use:

Systematics of photo lenses

There are no major differences in principle between camera lenses and lenses used in other devices. However, there are deviations in some details in the design and construction. Photo lenses can be interchangeable ( interchangeable lens ) or permanently attached to photo cameras.

According to angle of view

In the case of photo lenses, a further distinction is made according to the angle of view , which determines the focal length for a given image format:

Type typical angle of view typical focal lengths
(with 35 mm format, 36 mm × 24 mm, otherwise consider format factor )
purpose
Normal lens 40 ° to 55 ° 40 to 60 mm The image appears on normal 10 × 15 prints without the effects that are present with wide-angle or telephoto lenses.
Telephoto lens 2 ° to 35 ° 65 to 1200 mm (sometimes even more) Sports and nature photography , 85 to 100 mm are common for portraits , very long focal lengths for wildlife photography
Wide angle lens 63 ° to 114 ° 14 to 35 mm Reportage photography , wide scenes can be shown with a high depth of field; Landscape photography
Fisheye lens mostly 180 ° 8/16 mm (circular / full screen) captures the entire field of view with the distortion required for the angle of view ; artistic effects; Surveillance cameras , special applications such as the measurement of cloud cover or the coverage of the sky caused by tree leaves.
zoom lens differently Lenses are adjustable (e.g. 24 to 85 mm or 70 to 300 mm) Situations in which lens changes are cumbersome (e.g. bird watching ); Photography with compact equipment

Due to their variable focal length, zoom lenses are sometimes categorized according to the relative focal length range (e.g. zoom lens 1: 3 or 3x zoom , the zoom factor corresponds to the largest focal length divided by the smallest focal length). They are heavier and more expensive, the more luminous they are and the better imaging errors are corrected. There are also zoom lenses that are aimed at professional photographers and have the same (relatively small) f-number over the entire focal length range (e.g. 2.8) instead of the f-number increasing with increasing focal length (e.g. 4 to 5.6).

Basic constructions

An interchangeable lens with a very flat design, a so-called "pancake" (incl.
Lens hood ) mounted on a DSLR
The 30 cm lens (1: 2.5 / 75 cm) of the BMK 75 satellite camera from Zeiss Oberkochen

Construction of photo lenses

Sectional model of a lens.

A photographic lens can consist of a number of different elements. The most original form, consisting of one element, can be found in the box camera "Brownie" from Kodak . More complex zoom lenses can have more than 20 lenses, some of which are partially fixed to one another and further lenses can be moved relative to one another.

The lens positioned on the front is a key element in the performance of a photographic lens. This component is coated in all modern lenses to reduce abrasion, lens flare and surface reflections, as well as to regulate the color intensity. However, since such effects are often desired, the properties of camera lenses can be modulated again by adding special filters ( polarization filters , UV filters ). To avoid aberrations curvature is always selected so that the angle of incidence the angle of refraction equal, but this is not completely feasible with zoom lenses.

Because of its good optical properties, glass is widely used as a building material for lens systems. Other materials are quartz glass , fluorite , plastics such as plexiglass and also substances such as germanium or meteorite glass . Plastics allow the production of aspherical lens elements , which are very difficult to produce from glass and can simplify the handling of the lens. The outer lens elements in high-quality lenses are not made of plastic, as they are easier to scratch than glass.

The resolution of such systems is determined by the material used, the coating and the processing, and can be determined, for example, by the USAF chart . The resolution is limited by the diffraction , but there are very few (and very expensive) lenses that come close to the diffraction limitation. Modern lens systems are provided with numerous coatings in order to minimize undesired properties (e.g. UV coating).

The focus of the lens system is set by the distance between the lens and the object plane. A built-in camera system in some systems can adjust the distance between the systems while the lens is focusing on an object. This technology is named differently by the manufacturers (Close Range Correction, Floating System, Floating Lens Element etc.).

use

A projector uses a lens to enlarge a still or moving image onto a screen.

In a microscope or a telescope , the real image of very small or distant objects generated by the objective is viewed through an eyepiece , another lens system. In the microscope the objective has a short focal length compared to the eyepiece, in the telescope it has the longer focal length . In both cases, the image plane is close to the eyepiece.

The lens is part of cameras , digital and video cameras . It creates a real image in the image plane where the photosensitive film or an image sensor is located.

History and Development

After the use of pinhole cameras with glassless openings (see also camera obscura ), glass lenses have also been used for the production of real images since the 17th century. In order to improve the imaging properties of optical devices, objectives with suitable lens combinations have been developed.

Telescopic lenses

The search for high-performance lenses was initially determined by the needs of astronomy. The first lenses were one-piece collecting lenses made of glass and showed strong chromatic and spherical aberrations. There have been various advancements to eliminate or minimize this:

• the use of long focal lengths with small openings, such as the air telescope by Johannes Hevelius with a length of 45 m, mid-17th century,
• Around 1668 Newton's mirror lens , which due to the use of a concave mirror, has no chromatic aberration due to its principle . At the beginning of the 18th century, the brothers John, George and Henry Hadley succeeded in correcting the spherical aberration in mirror lenses by using a parabolic instead of the much easier spherical surface.
• also at the beginning of the 18th century the development of achromatic lenses , two interconnected lenses made of different types of glass, which completely corrects the chromatic aberration at two wavelengths and minimizes it in the surrounding area. This combination of two lenses also minimizes the spherical aberration.

The manufacture of large achromatic lenses was only possible at the beginning of the 19th century. In the late 19th century, then telescopic lenses were built with lenses with a diameter of up to one meter reflecting telescope lenses with a diameter of almost 2 meters.

Around 1900, Karl Schwarzschild investigated aberrations in telescope lenses, his analyzes led George Willis Ritchey and Henri Chrétien to the mirror configuration named after them , which minimized the most dominant aberrations and allowed observations with a larger angle of view . This configuration served as the basis for many modern reflecting telescopes, up to a diameter of approximately 10 meters.

Microscope objectives

Modern, apochromatic microscope objective.

Microscopes , made up of an objective and an eyepiece, had been known since the beginning of the 17th century, but in terms of image quality they were inferior to simple microscopes similar to a magnifying glass. This changed with the availability of new types of glass at the beginning of the 19th century, with which Joseph von Fraunhofer and others developed the first chromatically corrected lenses . At the end of the 19th century, Otto Schott succeeded in developing types of glass with which he manufactured an apochromatic lens corrected for three wavelengths.

The simplest representatives of the microscope objectives are the achromats optimized for two wavelengths, followed by the apochromats, objectives with a leveled field of view, e.g. B. for microphotography the plan achromatic lenses. The most complex and expensive lenses are plan apochromats, which can easily accept mid four-digit prices. Different intermediate stages are, for example, with special glasses, such as. B. built the Fluotars made with fluorite glass. There are also different types of construction for different applications or contrasting methods. Incident and transmitted light objectives with integrated rings for phase contrasting or objectives with glasses mounted without tension for polarization processes.

Important information that can be marked on the objective is the manufacturer, objective class, scale, numerical aperture , cover glass thickness, (mechanical) tube length, contrast method and others. A label according to

PlanC
40 × / 0.30
∞ / 0.17

therefore characterizes a planchromatic objective with 40x magnification and a numerical aperture of 0.3. The tube length is set to infinity and the cover slip correction to 0.17 mm (standard thickness). A designation of the species

100 × / 0.80 / Oil / Ph3
160 / -

indicates an oil immersion objective with 100x magnification and numerical aperture 0.8, which is suitable for phase contrast with ring size 3. The length of the tube would be 160; it is insensitive to cover slip defects.

Photography lenses

Lens of a medium format travel camera from the 1930s

The advancement of the lenses used made a decisive contribution to the advancement of photography in the second half of the 19th century. In the early days of photography, simple achromatic lenses were used , which had to be dimmed down to achieve sharp images or which only had a small aperture (largest aperture 1:16). Because of this weakness of light and the low sensitivity of the recording material at the time, very long exposure times resulted , which sometimes required the use of auxiliary devices to “keep the person depicted” when taking pictures.

Therefore, a great advance for the production of portraits was the invention of Petzvalobjektivs in 1840, a portrait lens of the Vienna physicist Josef Petzval . The bright lens (largest opening already 1: 3.6) consists of two double lens systems. It enabled portraits with the required short exposure time and had a favorable angle of view of 20 ° (light telephoto lens). The use of mathematical processes during the lens design was also trend-setting with Petzval's portrait lens. For example, Ludwig Seidel later investigated the aberrations of the lenses and in 1866 published a formula system that made lens construction easier.

For landscape and architecture shots, which were less about high light intensity and more about a large angle of view, small apertures were still used ; Around 1890, for example, Zeiss and Goerz brought designs with the largest openings of a maximum of 1: 6.3 or 1: 7.7 on the market. Around 1860 some special lens constructions were developed for such purposes, the first was probably by Thomas Sutton in 1858 with a 120 ° angle, soon followed by Hugo Adolph Steinheil with a periscope consisting of symmetrical menisci , which he improved to an aplanat a short time later . The Hypergon , developed at the end of the 19th century, had a similar construction , which consists of two menisci of the same surface curvature, has an angle of view of 135 ° and has a slight curvature of field .

In the following years, derived from the lens triplet and the symmetrical constructions (see Gaussian double objective), a whole series of objectives were developed. Sharpness, image quality and light intensity were mostly significantly improved. Paul Rudolph at Zeiss constructed the first anastigmatic lens (maximum aperture 1: 6.3) with the Protar lens in 1890. After the turn of the century, the speed of the lenses could be increased considerably. The first really fast lens with which one could take pictures by hand indoors without additional lighting, albeit due to the advances in the light sensitivity of the negative material, was probably the Ernostar with an aperture of 1: 2, sold from 1924 , later 1: 1.8. Other manufacturers offered even better values ​​around 1930 (Zeiss Sonnar, 1: 1.5 or 1: 2, Leitz Hektor, 1: 1.9, Leitz Summar 1: 2 and the Astro-Berlin speedometer with initially 1: 0, 95).

For a long time, it was customary to limit the lens to four groups. A higher number of groups was not advisable due to the reflections occurring on the glass surfaces. Any reflective glass surface reduces the amount of light that reaches the photographic layer. Part of the multiple reflected light also arrives on the photographic layer, but in the wrong place, and thus reduces the contrast of the image. A breakthrough came with the coating of the lenses with anti-reflective layers , which was developed in 1934 by Alexander Smakula at Zeiss. This paved the way for multi-lens lenses in which image errors are minimized, such as the Superachromat , which is corrected as a telephoto lens for four wavelengths and delivers sharpness up to the diffraction limit . Advances in computer technology have made it easier to calculate such complex optics since the early 1960s (like the OPREMA built in 1955 in the GDR ).

Since then, and in the meantime, a number of special lens designs have been developed. In the 19th century, concepts for zoom lenses were discussed in which the focal length can be adjusted. The first product was the Bell and Howell Cooke "Varo" 40–120 mm for 35 mm film cameras, 1932. With these lenses, too, the image quality was reduced in the course of time the years improved. Because of their flexibility, they were also used for photography since 1959. Luminosity and focal length range have been improved since then. In the meantime (2008) professional lenses with a focal length ratio of 1: 100 and an initial aperture of 1.7 are available for HDTV cameras.

Another lens design are so-called retrofocus wide-angle lenses , which have been used for film cameras since 1931 and for single-lens reflex cameras since 1950.

In modern digital camera systems , some relatively bright and high-quality standard zoom lenses with a typical zoom factor of around three have achieved a quality that hardly differs from the quality of lenses with a fixed focal length. In addition, there are increasingly zoom lenses with a relatively large zoom range, which are referred to as travel zoom lenses or super zoom lenses .

Focal length

Lenses whose focal length corresponds approximately to the diagonal of the respective recording format are referred to as normal lenses . You have an angle of view of around 53 degrees. In the 35mm format (digitally referred to as full format), to which focal length specifications often refer or are converted, the diagonal is 43.3 mm. Lenses with a smaller focal length and a larger angle of view are called wide-angle lenses , while lenses with a larger focal length and smaller angle of view are called long-range or telephoto lenses .

Focal length and perspective

When taking pictures with different focal lengths from the same location, there is no change in perspective, only a change in the image scale . A detail enlargement of one of the adjacent wide-angle photos would show exactly the same perspective as the corresponding image taken with a longer focal length. However, the range of depth of field changes .

In the case of recordings with different focal lengths but the same image scale, the perspective representation of the object changes as a result of the different recording distance . It can be clearly seen that the foreground of the object (a photo lens) is strongly emphasized when taking a picture with the wide-angle lens. When shooting with the telephoto lens, on the other hand, the background is more emphasized. However, this effect is not directly due to the different focal lengths. It is created by maintaining the same image scale when using different focal lengths. This in turn requires different distances from the object, which ultimately change the perspective. In favor of a large angle of view or a small distance, the perspective appears unnatural when using wide-angle lenses. This is particularly noticeable when taking portraits . With a wide-angle lens, the parts of the face close to the camera - often the nose - are displayed disproportionately large. With a light telephoto lens - a small image equivalent focal length of 80 mm - the portrait appears more natural.

Focal length specifications for DSLR and compact cameras

Actual focal length of a digital bridge camera engraved on the front lens mount, additional small image equivalent focal length information on the mount ("Equiv.135")

In the case of lenses for compact cameras or digital single-lens reflex cameras ( DSLR ) with a small recording format, the small image equivalent focal length is occasionally also specified (“Equiv.135” - the number is an often used nomenclature for naming 35 mm still images. “35” means moving image with 35mm film and “135” still image with 35mm film). It corresponds to the focal length of a 24 mm × 36 mm miniature camera that records the same angle of view (see main article format factor ).

Projection lenses

The development of projection lenses has followed two different lines in recent years. The traditional projection lenses are used to depict a template on a screen , including a canvas using light (" projection "). Optical projection lenses are used in particular in

These projection lenses are - with all their constructive peculiarities - closely related to the lenses for photography. In addition to slide and film projectors, enlargers for photography also use projection lenses. Most projection lenses in miniature - slide projectors are used are of structurally close relatives of Cooke triplets (eg. Meyer optics - Diaplan , Leitz - Elmaron , Will-Wetzlar - Maginon ). There are also more complex four (e.g. Ed.-Liesegang-oHG - Sankar , Leitz- Hektor ) or five-lens (Leitz- Colorplan ) projection lenses. Double anastigmats were also used earlier (e.g. Helioplan from Meyer-Optik). In addition to projection lenses with a fixed focal length, there are also those with a variable focal length ( zoom function).

The aperture ratios of projection lenses for slide projectors are now usually 1: 2.5 to 1: 2.8 for smaller rooms (focal length approx. 85–120 mm). The aperture is reduced to 1: 4 for larger rooms. In comparison, projection lenses for film projection usually have a significantly higher light intensity.

In the last few decades, new technical tasks for projection have developed. The photolithographic structuring of integrated circuits , which requires highly specialized optical systems, plays a special role . The projection takes place here with lasers , for which lenses with the highest imaging performance were created. In order to be able to image ever finer structures, lasers of short wavelengths are used (2008: 193 nm), for whose light only quartz glass is sufficiently transparent .

literature

Wiktionary: Objective  - explanations of meanings, word origins, synonyms, translations

Individual evidence

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6. Floating element ( Memento of the original from August 10, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
7. Wikisource: Men of Invention and Industry / Chapter XII
8. J. Paul Robinson, Purdue University's Department of Basic Medical Sciences: The Principles of Microscopy ( Memento of 25 June 2010 at the Internet Archive ) February of 2004.
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16. Data sheet XJ100 × 9.3 B 9.3-930 AF 1: 1.7 ( Memento of the original from May 14, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.4 MB), Canon
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