The requirements of a lens for enlarging are much the same as those for process work, except that the correction should be made for an enlargement of 2 to 15 times linear.
A lens for photo-topography (photography applied to topographic survey work) should embrace a very wide angle and give a perfect image of distant objects (corrections therefore limited to the focal plane) and, especially, absolute freedom from distortion and curvature of the field. For work executed from terrestrial stations, a small aperture is sufficient, but for aerial photography, where large apertures are requisite, the angle of field is usually smaller, and the correction for achromatism may be a simpler matter, since a yellow filter, which is always necessary in this class of work, may be permanently incorporated with the lens.
95. Lens Names. Before describing some characteristic types of lenses it may be useful to indicate the significance of some of the names by which manufacturers define certain qualities or properties of their instruments.' From the property of giving a " stigmatic " (§ 48) image, we get (by the addition of prefixes generally derived from the Greek), the names aristostigmat (best stigmatic), holostignat (com pletely stigmatic), isostigrnar (uniformly stig matic), orthostigmat (stigmatic without distor tion), neostigmar (a new anastigmat), planastig mat (anastigmat with flat field), velostigmat (rapid stigmat). Aplanatism of the image (§ 47) is suggested by the appellations aplatiastigmat, aplanat, aplanatic, antiplanat. A wide angle is suggested by the names eurygraphe, eurynar, euryplan (eurus = wide), perigraphe, periplan (per around), eurygonal, hypergon, teragonal (gone = angle, hyper = excess, teras = extra ordinary). Freedom from distortion should be the leading quality of rectilinear, rectigraph, rectoscope (rectum = straight), orthor, orthoplastic orthoscopic (orthos = correct), linear, collinear (line for line), homocentric (same centre of pro jection), ale/liar (ale/lies = true), verax. Flatness of field seems to be suggested by the names aristoplan, planar, and analogous names. Uni versality of use seems claimed for the lenses polynar, polyplast, polycentric (poly = many), pantogonal, pantoscope (pan = all).
Lenses having an amplifier for the large scale photography of distant objects (tele = far) are designated under the general classification of telephotos; in this category are found telecentric, telinear, , and bis-telar (giving an image twice the size of that given by a normal lens with the same camera extension).
Some lenses are named from the number of constituents (or groups of cemented lenses) : octanar, sextar, tetrastigmat, tessar (tessares = four), trianar, trioplane, triosymnietric, triotar, triplane (tri = three) ; others take their names from some peculiarity of construction : unofocal (the four component lenses have the same focal length), primoplane (perfect correction in the " primary plane "). Some names imply the idea of rapidity, e.g. eel or, others of luminosity, clarior, glaukar, leukar, lucidor, and all those named after a star or bright body, heliar, , hello gonal (hellos = sun), phoebus, stellor (stella = star), orlon, syrius, fulmenar (fulmen = light ning) ; others imply perfection, artar, or power, dynar. Some names are merely coined from the name of the manufacturer or from the initials, e.g.. dagor (doppel anastigmat Goerz) ; others bear fancy names the originality of which is often their sole point of interest.
No optician of repute sends out a lens without his name or trade-mark, which is an indispen sable guarantee to the buyer of a lens.
96. Brief Description of the Principal Types of Lenses. In the following paragraphs are briefly described some types of lenses, chosen as those which have marked an epoch in the development of photographic optics, with an indication of their essential characteristics, but without claiming to be a complete list, on ac count of the extremely wide variety of combina tions in use and the continual improvements being made. To facilitate comparisons, all the sectional drawings (Figs. 65 to 86), reduced to their essential elements, have been made on a uniform scale corresponding to lenses of 4 in. focal length. The principal types of optical glass are indicated by conventional shading, the key to which is given in Fig. 64. The stop drawn corresponds approximately to the maximum aperture.