filter, in photography, device used to selectively modify the component wavelengths of mixed (e.g., white) light before it strikes the film. Filters may be made of coloured glass, plastic, gelatin, or sometimes a coloured liquid in a glass cell. They are most often placed over the cameralens but can in some cases be placed over the light source with the same effect.
Black-and-white films are imperfect in their colour sensitivity, and coloured filters are used to modify the light and translate the subject into gray tones that correspond to the tones seen by the human eye. Coloured filters can also brighten or darken the reproduction of coloured objects, permitting local contrast controls at the point of exposure. In colour photography, coloured filters are used to alter the colour quality of the light to match the colour sensitivity of the film.
Some light filters are used in both colour and black-and-white photography. Neutral density filters decrease the intensity of the light without affecting its colour and are used when the light intensity is too great for the correct exposure. Polarizing filters enhance colour vividness by reducing glare from the reflecting surfaces of such substances as glass and water. Colour filters are also used for colour correction in the printing process and for selecting contrast scales of multicontrast black-and-white enlarging paper.
Since filters absorb some of the light that passes through them, an increase in the calculated exposure is usually required. This increase is known as the filter factor. Modern cameras with built-in meters measure the light after the filtration and thus take the decrease in intensity into account.
This article was most recently revised and updated by Amy Tikkanen.
images formed by convex and concave lensesRay diagrams show the types of images formed by convex and concave lenses. The characteristics of the image formed by a convex lens depend on the location of the object. In these diagrams, F is the focal length of the lens, and 2F is twice the focal length of the lens.
lens, in optics, piece of glass or other transparent substance that is used to form an image of an object by focusing rays of light from the object. A lens is a piece of transparent material, usually circular in shape, with two polished surfaces, either or both of which is curved and may be either convex (bulging) or concave (depressed). The curves are almost always spherical; i.e., the radius of curvature is constant. A lens has the valuable property of forming images of objects situated in front of it. Single lenses are used in eyeglasses, contact lenses, pocket magnifiers, projection condensers, signal lights, viewfinders, and on simple box cameras. More often a number of lenses made of different materials are combined together as a compound lens in a tube to permit the correction of aberrations. Compound lenses are used in such instruments as cameras, microscopes, and telescopes.
Optical principles for lenses
common lenses(Left) Cross sections of standard forms of common lenses. (Right) Refraction of light by converging and diverging lenses, showing the principal axis, the principal focus (or focal point) F, the focal length f, and the focal plane.
A lens produces its focusing effect because light travels more slowly in the lens than in the surrounding air, so that refraction, an abrupt bending, of a light beam occurs both where the beam enters the lens and where it emerges from the lens into the air.
A single lens has two precisely regular opposite surfaces; either both surfaces are curved or one is curved and one is plane. Lenses may be classified according to their two surfaces as biconvex, plano-convex, concavo-convex (converging meniscus), biconcave, plano-concave, and convexo-concave (diverging meniscus). Because of the curvature of the lens surfaces, different rays of an incident light beam are refracted through different angles, so that an entire beam of parallel rays can be caused to converge on, or to appear to diverge from, a single point. This point is called the focal point, or principal focus, of the lens (often depicted in ray diagrams as F). Refraction of the rays of light reflected from or emitted by an object causes the rays to form a visual image of the object. This image may be either real—photographable or visible on a screen—or virtual—visible only upon looking into the lens, as in a microscope. The image may be much larger or smaller than the object, depending on the focal length of the lens and on the distance between the lens and the object. The focal length of a lens is the distance from the centre of the lens to the point at which the image of a distant object is formed. A long-focus lens forms a larger image of a distant object, while a short-focus lens forms a small image.
Usually the image formed by a single lens is not good enough for precise work in such fields as astronomy, microscopy, and photography; this is because the cone of rays emitted by a single point in a distant object is not united in a perfect point by the lens but instead forms a small patch of light. This and other innate imperfections in a lens’s image of a single object point are known as aberrations. To correct such aberrations, it is often necessary to combine in one mount several lens elements (single lenses), some of which may be convex and some concave, some made of dense high-refractive or high-dispersive glass, and others made of low-refractive or low-dispersive glass. The lens elements may be cemented together or mounted at carefully calculated separations to correct the aberrations of the individual elements and obtain an image of acceptable sharpness (see alsoaberration). The precise mounting also ensures that all lenses are properly centred; that is, the centres of curvature of all the lens surfaces lie on a single straight line called the principal axis of the lens. A frequently used measure of the quality of any lens system is its ability to form an image that is sharp enough to separate, or resolve, two very close dots or lines in an object. Resolving power depends on how well the various aberrations in a lens system are corrected.
The simplest compound lens is a thin cemented combination of two single lenses, such as that used in the objective (the lens nearest the object) of a small refracting telescope. Microscope objectives may contain as many as eight or nine elements, some of which may be made of different materials in order to bring all colours of light to a common focus, and thus prevent chromatic aberration. The objective lenses used in cameras may contain from two to 10 elements, while a so-called zoom or variable-focal length lens may have as many as 18 or 20 elements in several groups, the different groups being movable along the axis by levers or cams in order to produce the desired change in focal length without a shift of the focal plane. Lenses also vary greatly in diameter, from as small as 0.16 cm (1/16 inch) for an element in a microscope objective to as large as 100 cm (40 inches) for an astronomical telescope objective. In reflectors and several other types of astronomical telescopes, concave mirrors are used for the objective instead of lenses.
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