This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
By Taylor, Taylor & Hobson, Ltd., New York City.
829. The Cooke Anastigmats differ from others, in that the lens consists of three glasses, and with this simple construction is combined a unique screw adjustment for use in making final corrections. Errors which remain in more complex systems are thus easily removed and a uniform excellence is attained, whereas the older anastigmats frequently show a marked difference one from another. Obviously, more light reaches the sensitive plate through three glasses than through six or eight. Moreover, there is no cement, and the adjustable air spaces assist still further in correcting the entire system. Since the first appearance of the lenses, European opticians have recognized that while mathematically perfect, these are mechanically the most simple invented since the introduction of Jena glass. They are light, compact, and rigid and durable to a remarkable degree.
830. These statements give reason why Cooke lenses are employed in astronomical observatories, and why they are used at Harvard. For difficult copying and enlarging they are invaluable. They are now used exclusively by the U. S. Geological Survey, and by other departments at "Washington. In process-engraving works, throughout Europe and America, Cooke lenses are used under process-gratings of 250 lines or more to the inch. For tricolor-photography they are unique, because the screw adjustment gives an exact co-incidence in the sizes of the color-images.
831. While thus selected for the best scientific work,
Cooke lenses are used by amateurs everywhere, by engineers, in the portrait studio, by army and navy photographers, and by newspaper men who seek, above all else, rapidity and fine definition. Wherever possible, the makers facilitate the trial of Cooke lenses for comparison with others.
Cooke Extension Lenses. By removing the back-glass and substituting another, the entire focal length is increased. Thus, from the same point of view the photographer obtains larger images of distant objects. These extension lenses increase the size of image about 50 per cent. For example: An object taken with the normal lens, two inches long in the photograph, is, from the same position, made three inches long with the extension lens. Better results are thus obtained than with portions of other types used alone. The normal Cooke lenses may be used upon
Cooke Lens, Series II. Full aperture F. 4.5 (See Illustration No. 75). These ultra-rapid anastigmats are designed for the finest portraiture, and for subjects demanding extreme speed. Like other Cooke lenses, they give definition at the margins of the plates equal to that at the center, and are quite free from that peculiar streakiness or marginal definition familiar to the professional photographer.
834. The lenses numbered 22 and 23 are provided with the means for moving the back-glass as shown in the illustration. This device enables the photographer to secure at will uniformly sharp definition, or to introduce any required softness evenly throughout the plate.
835. The right choice of a shutter is of vital importance. The highest efficiency in photographing rapidly moving subjects can be secured only with focal-plane shutters, while for portraiture a suitable studio shutter is best.