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.
Wide-Angle Lens. It should now appear quite clear to you that if a lens is to have great covering power its barrel must be short. In other words, the two glasses, the front and back combinations, must be mounted close together. If you compare an ordinary - especially an old pattern - rapid rectilinear lens with a wide-angle rectilinear, you will find this is what the optician does. The close mount tends to increase the curvature of field, however, and in order to prevent the photographer using the lens at too large an aperture and then condemning it for poor marginal definition, the stop F. 16 is usually the largest one provided. Most of the modern anastigmats have their combinations mounted close together in a short barrel, consequently have great covering power; but as with them both astigmatism and curvature of field are practically eliminated, the larger aperture of F. 8, or even F. 6, is available.
664. What Plate Will the Lens Cover? - Going back to the 3 1/4 x 4 1/4 lens you may measure the diameter of the circle of illumination on the ground-glass of the 12 x 15 camera. Supposing this to be 8 inches, draw a circle of this diameter on a sheet of paper. The lens will then cover any plate that can be laid inside this circle. A 5 x 7 plate will extend beyond it at each corner, so the lens will not cover a 5 x 7 plate. The 4x5 plate will go inside the circle, as will a 3 1/4 x 4 1/4 plate, the lens covering either of these sizes easily. Now, raise the front of the 12 x 14 camera and note that the circle of illumination on the ground-glass is raised at the same time. It will be quite evident, then, that there must be an excess of covering power so that you can adjust the image on the plate, the position of which in your camera is fixed by raising or lowering the front carrying the lens. In cases where none of this excess or reserve of covering power exists, raising the lens frequently produces dark corners in the sky of the picture.
The Focal Length. With the older types of lenses the focal length had much to do with relation to covering power, and it is with this type of lens that the focal length must be not less than the diagonal of the plate. If shorter than this (about 5 inches for 3 1/4 x 4 1/4 plate, 6 inches for 4 x 5 plate, and 9 inches for a 5 x 7 plate), there will be insufficient covering power, and shorter foci lenses of the wide angle type are necessary. With modern lenses, however, there is, as has already been said, a reserve of covering power. The lens may be chosen of a suitable focal length with reasonable certainty of covering the plate. Thus a 5 inch anastigmat, which may be the normal lens, with 3 1/4 x 4 1/4 camera, can be used on a 5 x 7 plate with confidence. This could not be done in the case of the rapid rectilinear. An approximate rule for determining the foci to be used on any size plate is, for a wide angle, the shorter dimension of the plate; for a normal angle, the diagonal, and for a narrow angle or long focus lens, the length and width of the plate added together. In the case of a 5 x 7 plate the three lengths would be approximately 5 inches, 8 1/2 inches and 12 inches focus, respectively.
666. Schriever Universal Extra Rapid Lens, Speed F/6. - The production of this new lens is the result of a pressing demand for a lens that would perform the highest grade of service in all the departments of photography.
668. With it instantaneous pictures can be made on dark and misty days where slower lenses would utterly fail. The 4x5 and 5x7 sizes readily adjust to hand cameras. The larger sizes are highly recommended for studio portraiture and group work. With the 8 x 10 size, a full length cabinet picture can be made in a fifteen foot operating room. Their speed, depth of focus, compactness and high grade general utility at the moderate prices at which they are quoted, give them much prestige in the photographic world.
669. Owing to the fact that the front and back combinations of this lens are individually corrected, the operator may, by the use of the back combination alone, produce an image of twice the size of one obtained with the entire objective, and also increase the focal length of same to double that of the whole lens. These lenses are supplied fitted with Iris diaphragm. They are also furnished fitted complete with Unicum shutter. When equipped with shutter the Iris diaphragm is included in the shutter.