Early Work In Photography
A Text-book For Beginners

Facts about Lenses. 61 angle lens; while, if we used it for a plate twelve inches by ten, it would be a wide-angle. On the quarter-plate it would be a long-focus, while on the twelve by ten it would be a short-focus lens. The terms wide-angle, sliort-focus, etc., are relative, not absolute; and depend upon the focus of the lens in relation to the plate it is covering. The ordinary rectilinear lens has usually a focal length about equal to the diagonal of the plate it is intended to cover ; and a lens of focal lengtli appreciably longer than the diagonal of the plate, is called a long-focus or narrow-angle lens, for the two terms are interchangeable. From this it might seem that long-focus or short-focus lenses can be used indifferently and interchangeably, but this notion has its limits. While it is true that any nine- inch focus lens covering a 12 x 10 plate is a wide-angle, it is also true that many nine-inch focus lenses will not sharply cover a plate of that size. A rapid rectilineal is usually constructed to work with a stop as large as f/8; while the wide-angle usually has its largest stop f/113 Or 7/16 Although a wide-angle lens will always cover a smaller plate than tliat for which it is intended, a narrow-angle will not always cover a plate much larger than that for which it is made. And, although the wide-angle can always be used as a medium or narrow-angle on a plate smaller than that for which it is intended, it will not allow of such extremely rapid exposures as could be made with a medium, or narrow-angle lens.., All other things being equal, it is better to employ a medium-angle than a wide-angle lens; because, with the latter, the images of things near the edge of the ground- glass (or sensitive plate) are in a strained or distorted per- spective, for the reason explained in figs. 8 and 9. If it is necessary to have the image of a good size, and impossible to get far away from the subject; it is often absolutely necessary to use a very wide-angle lens., The instructions in this chapter, if once fully grasped, will give a fair working knowledge of the properties of the lens There are many more advanced points that must be left for a later book, and some of the applications will be treated in succeeding chapters.