LENGTHENING CAMERA For copying, and in photographing small objects at close quarters, the bellows extension of an ordinary camera sometimes proves insufficient. The professional photographer in such a case makes use of a wooden tube or cone, fitting on the front of the camera and carrying the lens at the outer end, as illustrated. But a good modem studio camera will usually have an extension adequate for any probable require ments. For smaller cameras, extension acces sories of various kinds are obtainable, some being made to fit the front and others the back of the camera. (See also " Extension, Camera.") LENS (Fr., Lentille, Loupe, Objects' f ; Ger., Objectiv, Linse) Photographically, a lens is a combination of two or more glasses capable of producing an photographic objective. The elementary forms of lenses, as illustrated under the headings "Concave Lens" and "Convex Lens," made in a great variety of glasses and with widely differing curves, are used in combination to build up the more or less complex objectives now in use. The simplest combination of these elements is found in the single " landscape lens " A, which is composed of a double convex lens of crown glass cemented to a plano-concave of flint glass, thus securing achromatism (see " Chromatic Aberration ") ; while the latest and most complex combination is found in the Zeiss anastigmat B, composed of four elements cemented together, two of such combinations forming the " Series VIIa Rapid Anastigmat." It will be convenient to deal with the principal types of lenses in groups, showing their gradual development, as improvements in the various forms have been and are proceeding simul taneously.
The action of a photographic lens may be better understood by considering what happens when a small bevelled opening is made in the shutter of -a darkened room, as at D. Rays of light from all parts of any object outside—say, a church—are admitted by the aperture, cross each other, and proceed in straight lines to form an inverted image on the wall opposite. Photo graphs can, in fact, be made with a pinhole instead of a lens. Except with a very small opening, however, which means a long exposure, the image is blurred. By using a convex lens, as at E, which has the property of converging light rays and bringing them to a focus, a much larger aperture becomes possible, together with improved definition.
Single or Landscape Lenses.—In its primitive
form, the single combination was nothing more image. Simple or " spectacle " lenses, con sisting of one piece of glass, are occasionally used to obtain special effects, but the single achromatic combination may be taken as the starting point in the evolution of the modern than the object lens of a field glass. The first lens made specially for photography was of this model, and was issued by Chevalier, of Paris. This maker soon issued an improved model, now generally known as an achrcimatic meniscus C, which had a much wider field of definition and greater rapidity than its predecessor. This was followed by Grubb's aplan atic F which departed from the telescope lens construction, being composed of two meniscus elements ; in this lens the relative positions of the crown and flint glasses were reversed, greater covering power and rapidity being thus concave flint. Both these early forms of portrait lenses were comparatively slow in action, and were superseded by the Petzval portrait lens K (introduced by Voigtlander in 1840), which, with little modification, is the standard lens for studio work at the present day. It gives greater obtained as well as portability. The next forward step was made when J. H. Dallmeyer constructed a wide-angle landscape lens of three elements (see G), a concave flint being between two crown meniscus glasses. This covered the widest angle ever attained by any single lens, the longest side of the plate being equal to the focal length of the lens, while the curvilinear distortion was reduced to a minimum. By the use of different glasses, T. R. Dallmeyer constructed d lens on the same lines, covering a narrower angle, but working at the large aperture of f/io. This was known as the Rapid Landscape lens (see H) and was recom mended for distant views, large heads, and subjects where pleasing perspective was pre ferable to wideness of angle. Still later, the same optician produced d non-distorting " single " lens, the " Rectilinear Landscape" I, which, although fitted with an outside diaphragm, was absolutely rectilinear. Its comparatively small aperture (Fig.) prevented its general adoption, the rapid rectilinear with an intensity of f/8 being preferred. There is an internal air-space in this lens. A somewhat similar lens was produced by J. T. Goddard, but did not appear on the market.