Construction of Photo Graphic Lenses

On the Continent the sequence of F. numbers is usually 3.16, 4.5, 9, 12.5, 18, 36, the exposure in each of the last seven cases being, as in the two preceding systems, double that which is found necessary in the case of the preceding num ber.

The fact that

depth of focus is conditioned not only by aper ture but also by focal length has an important bearing upon the construction of very large aperture lenses. It is obviously of small use to build a cumbrous and costly lens with a focal length of, say, 12 in. and an aperture of, say f/3 for ordinary work, in which the lens would almost invariably need to be stopped at any rate to f/8 to secure reasonable "depth." But, with the extended use of diminutive cameras carrying very short focus lenses, the demand for very wide apertures has grown enormously, and lenses working at f/3.5 are now common.

An

important consideration in the construction of a photo graphic lens is the angle of view it embraces. If over 50° it is usually described as wide, if under 35° as narrow. The angle depends on the focal length of the lens in relation to the size of the plate, and in practice the focal length should not be less than the diagonal of the plate. In the case of a half-plate this diagonal is 8 in., in that of a quarter-plate 5.3 in. Tables of view angles are published in the B.J.A. showing the number of degrees em braced according to the quotient arrived at by dividing the diagonal of the plate by the equivalent focus of the lens. In the case of an 8 in. lens used on a half-plate, the quotient arrived at being 1, the angle according to the table is 53°.

The term focus is explained in the general article on LENS (q.v.), but it may be pointed out here that the equivalent focus of a photographic lens must be measured from the nodal point of emergence which is sometimes in front of the front component.

The historical genesis of the principal forms of photographic lenses having already been discussed, their actual construction, apart from the optical considerations dealt with in the general article on LENS (q.v.), may now be glanced at. The single lens is seldom produced commercially to-day except as a component of a doublet or triplet, or in connection with the cheaper amateur outfits. In the latter case it is a simple achromatic meniscus corn

posed of a biconvex crown cemented to a biconcave flint. The single components of the better types of "Rapid Rectilinear" are constructed in a somewhat different manner. Anastigmat singles, such as those used to make up the combinations of the Zeiss "Protar," the Ross "Combinable," the Steinheil "Ortho stigmat," the Watson "Holostigmat" and the Voigtlander "Col linear," are usually triple compounds of special Jena or Chance glasses. In addition to the symmetrical doublets formed by the combination of two simple singles there are a number of vari ously composed asymmetrical doublets; some with cemented, others with air-spaced components, are also notable combinations which, although assembled in two cells, are more or less frankly triplets. The Zeiss Tessars, which are entirely asymmetrical, con sist of three separate components, that in front being a cemented lens, the other two uncemented, the intermediate one being a double concave. In the construction of most modern doublets and triplets air-spaces play an important part. (See LENS.) Mul tiple combinations, such as the Beck Isostigmar, in some forms of which five separate lenses are sometimes used, are uncommon.

One interesting development has been by Zeiss in his use of con verging achromatic eyepieces (consisting of two self-contained groups of lenses) for increasing marginal thickness of lens.

Apart from, or in connection with, their standard models some lens-makers have designed supplementary lenses to lengthen or shorten the foci of ordinary objectives. In their simplest form such lenses are either long focus negative or positive singles which can be attached to the front of an ordinary objective with, necessarily, some disturbance of the original corrections. As far back as 1894 an advance in this direction was made by Taylor, Taylor and Hobson in connection with their "Cooke" lens by producing a suitably corrected rear extension lens for increasing the focal length. More recently the same principle has been applied by the Zeiss firm to its Tessars in a series of "Distars" and "Proxars," the former lengthening, the latter shortening the focal lengths of the original objectives.