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.
111. While the wide-angle lens is at no time the lens which will give the most perfect representation of a building, scene or object, its use is often compulsory, especially in cramped positions or narrow quarters.
The Hypergon Lens. The shorter the focal-length of a lens, in proportion to the size of plate it is intended to cover, the greater the angle it cuts; in other words, the nearer it can be brought to the object to be photographed, and still retain absolute sharpness on the plate, the greater is the angle. The ordinary inexpensive wide-angle lens has an angle of from 75 to 90 degrees; anastigmat wide-angle lenses are made to cover an extreme angle of 110 degrees. Even at this angle, certain subjects come into the commercial man's sphere, which cannot be brought on to the plate, owing to extreme height, or unusually narrow quarters. To cover these extreme instances, the Hypergon lens was put on the market by the C. P. Goerz Company. This lens differs in construction from any other form of wide-angle lens, and is so calculated that it will cut the enormous angle of 135 degrees. What this means can be realized when one remembers that a full arc only measures 180 degrees. (See Illustration No. 13.)
Size Plate Covered. The diameter of the image circle - in other words, the diagonal of the plate covered by the Hypergon double anastigmat - is equal to five times its focal-length. This means that a Hypergon of 6-inch focal-length will cover a plate of 20 x 24 inches, thus fully covering four times as great an area as a 6-inch lens of the old construction. The peculiar construction of the lens, which consists of two semi-spherical single lenses, is such that the light falls off very rapidly toward the margins of the image. This is unavoidable in all forms of wide-angle lenses, but becomes more noticeable the greater the angle of the lens.
The Star-Diaphragm. To overcome this, the Hypergon is supplied with a rotating star-diaphragm, which is so delicately adjusted over the optical center of the lens that a pressure of air from a bulb attached to a small blowpipe, fixed on the mounting of the lens, starts it rotating. In this way the light is held back over the center of the picture during a part of the exposure. The diaphragm is then dropped by pressing a spring and the balance of the necessary exposure given. The plate is in this way given even illumination all over. A regular leather cap is supplied to cover the lens before and after exposure.
115. Under certain favorable light conditions the Hypergon can be used at a speed of 1-35 of a second. For this purpose a special form of shutter is supplied, similar to the old drop-shutter, but arranged with its opening so that a greater amount of light reaches the margin of the lens than the center, thus equalizing the exposure. Usually, however, the small stop is employed, using the rotating diaphragm, and an exposure of from 3 to 15 seconds given.