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TessarThe Zeiss Tessar is a famous photographic lens design conceived by Paul Rudolph in 1902.
The name Tessar derived from the Greek word tetra to indicate the typical four lenses scheme.
The Tessar is an evolution of the Cooke triplet design in which the rear element is replaced by a cemented achromatic doublet.
A Tessar comprises four elements in three groups, one positive crown glass element on the front, one negative flint glass element at the center and a negative flint glass element cemented with a positive crown glass element at the rear.
Early Tessar designs allowed a maximal aperture of f/6.3 but development allowed a maximal aperture of f/4.5 by 1917, further improvements allowed an aperture of f/3.5. The last evolution of the Tessar had a maximal aperture of f/2.8, but the picture quality at full aperture is significantly inferior to an f/3.5 Tessar at full aperture.
The Tessar design was patented by Zeiss for two decades but the design was licensed to Bausch & Lomb in the United States and to Krauss in France.
Image:Zeiss_Tessar.png
Category:Photographic lenses
Photographic lens
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A photographic lens (or more correctly, objective) is an optical lens 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 practice a compound lens made up of a number of optical lens elements is required to correct the many optical aberrations that arise.
There is no difference in principle between a lens used for a 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 and other properties.
A practical camera lens will often incorporate an aperture adjustment mechanism, often an iris diaphragm, to regulate the amount of light that may pass. A shutter, to regulate the time during which light may pass, may be incorporated within the lens assembly, or may be within the camera, or even, rarely, in front of the lens.
The lens may usually be focused by adjusting the distance from the lens assembly to the image-forming surface, or by moving elements within the lens assembly.
The lens elements are made of transparent materials. Glass is the most widely used material due to its good optical properties and resistance to scratching. Various plastics, such as acrylic (or PMMA), the material of Plexiglas, can also be used. Plastics allow the manufacture of strongly aspherical lens elements which are difficult or impossible to manufacture in glass, and which simplify or improve lens manufacture and performance. Plastics are not used for the outermost elements of all but the cheapest lenses as they scratch easily. Moulded 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".
The maximum usable aperture of a lens is usually specified, as the focal ratio or f-number, the focal length divided by the actual aperture in the same units. The lower the number, the more light is admitted through the lens. Practical lens assemblies may also contain mechanisms to do with measuring light, to hold the aperture open until the instant of exposure to allow SLR cameras to focus with a bright image, etc.
The two main optical parameters of a photographic lens are the focal length and the maximum aperture. The focal length determines the angle of view, the size of the image relative to that of the object, and the perspective; the maximum aperture limits the brightness of the image and the fastest shutter speed usable.
Focal lengths are usually specifed in millimeters (mm), but older lenses marked in centimeter (cm) and inches are still to be found. For a given film or sensor size, specifed by the length of the diagonal, a lens may be classified as
- Normal lens: angle of view of the diagonal about 50°, the same as the human eye: a focal length approximately equal to the diagonal produces this angle.
- Wide-angle lens: focal length shorter than normal, and angle of view wider.
- Long-focus or telephoto lens: focal length longer than normal, and angle of view narrower. A distinction is sometimes made between a long-focus lens and a true telephoto lens: the telephoto lens is designed to be physically shorter than its focal length.
The 35mm film format is so prevalent that a 90mm lens, for example, is always assumed to be a moderate telephoto; but for the 7x5cm format it is normal, while on the large 5x4 inch format it is a wide-angle.
The real difference between lenses of different focal length is not the image size, but the perspective. You can take photographs 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 your distance from the subject. But the perspective will be different. With the wideangle, the hand will be exaggeratedly large relative to the head; as the focal length increases, the emphasis on the outstretched hand decreases. However, if you take pictures from the same distance, and enlarge and crop them to contain the same view, the pictures will be truly identical. A moderate long-focus (telephoto) lens is often recommended for portraiture because the flatter perspective is considered to look more realistic.
Some lenses, called zoom lenses, have a focal length which varies as internal elements are moved, typically by rotating the barrel or pressing a button which activates an electric motor. 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.
The complexity of a lens—the number of elements and their degree of asphericity—depends upon the angle of view and the maximum aperture. 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 (with 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 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 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, zoom lenses in particular, 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.
Special purpose photographic lenses
- Macro lenses are designed for good performance at close distances, e.g., for images of the same size as the object.
- Apochromat lenses have extreme correction for aberrations of colour.
- Process lenses have extreme correction for aberrations of geometry (pincushion distortion, barrel distortion).
Process and apochromat lenses are normally of small aperture, and are used for extremely accurate photographs of static objects.
- Enlarger lenses are made to be used with photographic enlargers (specialised projectors), rather than cameras.
- Lenses for aerial photography
- Fisheye lenses: extreme wide-angle lenses with an angle of view of 180 degrees, with very noticeable 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 for photographing the vain
- Infrared lenses
- Ultraviolet lenses
Some notable photographic optical lens designs are:
- Angenieux retrofocus
- Cooke triplet
- Double-Gauss
- Leitz Elmar
- Rapid Rectilinear
- Zeiss Tessar
- Zeiss Sonnar
- Zeiss Planar
Some lens manufacturers (2005):
- Canon
- Cosina
- Konica Minolta
- Leica
- Nikon
- Pentax
- Tamron
- Tokina
- Schneider Kreuznach ([http://www.schneider-kreuznach.com])
- Sigma Corporation
- Zeiss
See also
- Lens (optics)
- teleconverter
- Lens hood
- Large format lens
Category:Photographic lenses
ja:写真レンズ
th:เลนส์ถ่ายภาพ
Cooke tripletThe Cooke triplet is a photographic lens design designed and patented in 1893 by Dennis Taylor who was employed as chief engineer by Cooke of York. It was the first lens system that allows elimination of most of the optical distortion or aberration at the outer edge of lenses.
A Cooke triplet comprises a negative flint glass element in the centre with a crown glass element on each side. In this design, the sum of all the curvatures times indices of refraction can be zero, so that the field of focus is flat. In other words, the negative lens can be as strong as the outer two combined, when one measures in diopters. Yet the lens will converge light, because the rays strike the middle element close to the optic axis. The curvature of field is determined by the sum of the diopters, but the focal length is not.
It was at that time a major advancement in lens design. The triplet design was made obsolete by later designs on high end cameras, but remained widely used up to this day on cheap cameras.
Despite the fact that the Cooke design was patented in 1893 it seems that the use of achromatic triplet designs in astronomy appeared as early as 1765. The 1911 encyclopedia Britanica wrote:
:The triple object-glass, consisting of a combination of two convex lenses of crown glass with a concave flint lens between them, was introduced in 1765 by Peter, son of, John Dollond, and many excellent telescopes of this kind were made by him.
A similar design is used in the strong focusing synchrotron, invented first by Nicholas Christofilos in 1949, but his work was not known in the U.S., where parallel development took place.
See also
- Chromatic aberration
External links
- [http://wwwrsphysse.anu.edu.au/admin/php/fire_in_the_belly/Fire_in_the_Belly03.pdf Strong focussing synchrotron]
Category:Photographic lenses
Crown glassCrown glass is either of two kinds of glass.
One type comes from the way window glass was first made. Glass was blown into a crown or hollow globe that is then flattened and cut. It is flattenned by reheating and spinning out the bowl-shaped piece of glass (bullion) into a flat disk by centrifugal force, up to 5 or 6 feet in diameter. The glass is then cut into the size required, because of the manufacturing process the best, thinnest, glass is in a band at the edge of the disk, with the glass becoming thicker and more distorting towards the centre. Due to the distribution of the best glass, in order to fill large window spaces many small diamond shapes would be cut from the edge of the disk and these would be mounted into a lead lattice work and fitted in the window.
Crown glass was one of the two most common processes for making glass for windows up until the 19th century, the other was blown plate. The process was first perfected by French glassmakers in the 1320s, notably around Rouen. The process was kept a careful trade secret, for example the first crown glass made in London was not until 1678.
Crown glass is one of many types of hand blown glass. Other methods included: Broad sheet, Blown plate, Polished plate and Cylinder blown sheet. These methods of manufacture lasted at least until the end of the 19th Century. The early 20th Century marks the move away from hand-blown to machine manufactured glass such as Rolled Plate, Machine drawn cylinder sheet, Flat drawn sheet, Single and Twin ground polished plate and Float glass.
The second use of the phrase is for a type of optical glass used in compound lenses. This Crown glass is produced from a special alkali-lime silicate incorporating a high amount of phosphorus pentoxide, it has a low refractive index and low dispersion (high Abbe number).
The first recorded use of crown glass (in the UK) in windows was at the Banqueting House at the Palace of Whitehall, London, in 1685.
Category:Glass
Category:Optical materials
Flint glassFlint glass is an 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 1.45-2.00. A concave lens of flint glass is commonly combined with a convex lens of crown glass to produce an achromatic doublet lens because of their compensating optical properties.
With respect to glass, the term "flint" derives from the flint nodules found in the chalk deposits of southeast England that were used as a source of high purity silica by George Ravenscroft, circa 1662, to produce a potash lead glass that was the predecessor to English lead crystal.
Traditionally, flint glasses contain around 4%—60% lead oxide, however the manufacture and disposal of these glasses are sources of pollution. In many modern flint glasses, the lead can be replaced with other additives such as titanium dioxide and zirconium dioxide without significantly altering the optical properties of the glass.
Flint glass can be fashioned into rhinestones which are used as diamond simulants.
See also
- Chromatic aberration
References
- Kurkjian, Charles R. and Prindle, William R. (1998). Perspectives on the History of Glass Composition. Journal of the American Ceramic Society, 81 (4), 795-813.
Category:Glass
Vik MunizVik Muniz is an avant-garde artist who experiments with novel media. For example, he made two detailed replicas of the Mona Lisa: one out of jelly and the other out of peanut butter. He has also worked in sugar, wire, thread, and chocolate sauce.
More recently he has been creating larger-scale works, such as pictures carved into the earth (geoglyphs) or made of huge piles of junk. His sense of humor comes through in his "Pictures of Clouds" series, in which he had a skywriter draw crude outlines of clouds in the sky.
External links
- [http://www.vikmuniz.net/ Official web site]
- [http://www.pbs.org/independentlens/worstpossibleillusion/ PBS Independent Lens feature]
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