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Revision as of 20:30, 4 March 2007
A photographic lens (also known as objective 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.
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.
Construction
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, 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 1951 USAF Resolution Test Chart is one way to measure the resolving power of a lens in a standardized manner. The quality of the lens material, of the coatings and of the internal manufacture all influence the "resolution" of the lens, even at the same F-number and focal length. Today, most lenses are multi-coated in order to minimize lens flare and other unwanted effects. Premium lenses have a UV coating to keep out the ultraviolet light that could expose photosensitive materials and result in inaccurate colors.
Aperture and focal length
The maximum usable aperture of a lens is usually specified as the focal ratio or f-number, which is equal to the focal length divided by the effective aperture (or entrance pupil) diameter in the same units. The lower the number, the more light per unit area is delivered to the focal plane. Practical lens assemblies may also contain mechanisms to deal 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, and the size of the image relative to that of the object; the maximum aperture limits the brightness of the image and the fastest shutter speed usable.
Focal lengths are usually specifed in millimetres (mm), but older lenses marked in centimetres (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°: 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 uses a telephoto group to be physically shorter than its focal length.
The 35mm film format is so prevalent that a 90mm lens, for example, is sometimes assumed to be a moderate telephoto; but for the 7×5cm format it is normal, while on the large 5×4 inch format it is a wide-angle.
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 flatter perspective corresponding to the longer shooting distance is considered to look more flattering.
Number of elements
The complexity of a lens—the number of elements and their degree of asphericity—depends upon the angle of view and the maximum aperture, among other things. 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.
Zoom lenses
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.
Lens mounts
Most Single-lens reflex cameras, and some rangefinder cameras have detachable lenses. The lenses attach to the camera using a lens mount, which often also contains mechanical or electrical linkages between the lens and camera body. The lens mount is an important issue for compatibility between cameras and lenses; each major camera manufacturer generally has their own lens mount which is incompatible with others.
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 (APO) lenses have extreme correction for chromatic aberration.
- 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 up to 180 degrees or more, 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 and have an imperfection-removing effect popular among portrait and fashion photographers.
- Infrared lenses
- Ultraviolet lenses
- Perspective correction lenses (also called "tilt and shift lenses"), mainly used in architectural photography to mitigate the effects of perspective
Notable photographic optical lens designs
Some notable photographic optical lens designs are:
- Angenieux retrofocus
- Cooke triplet
- Double-Gauss
- Goerz Dagor
- Leitz Elmar
- Rapid Rectilinear
- Zeiss Tessar
- Zeiss Sonnar
- Zeiss Planar
Some lens manufacturers (2006):
- Canon
- Cosina
- Leica
- Minolta
- Nikon
- Olympus
- Pentax
- Tamron
- Tokina
- Schneider Kreuznach
- Sigma Corporation
- Zeiss