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.
Fixed Focus. This term applies chiefly to small box cameras or to those fitted with short focus single lenses. It does not mean any distinct kind of a lens with extraordinary power of covering objects near or far in one plane. The lens is only a fixed focus when it is immovable. While any lens can, practically speaking, be made a fixed focus, yet its length of focus determines the extent it will focus sharp all objects on one plane. Therefore, those only of very short focus can be used for this purpose, and only small pictures are satisfactorily made with fixed focus cameras. With them, the difference in focus of any object, far or near, is so little that it is not noticeable in the picture. On the larger sizes, however, it would be quite visible. There is no altering of the focus in a fixed focus box camera; the lens in the camera is stationary.
Fixed Focus Folding Camera. In the case of the fixed focus bellows camera, the bellows extends to its full length and locks. It is then, practically speaking, a fixed focus camera, the same as a box camera, for there is no adjusting of the bellows. Hence the term, fixed focus folding camera.
Universal Focus. The term Universal Focus is quite frequently applied to adjustable focusing cameras. We speak of the Universal Focus when the pointer is set at 100 feet on the focusing scale. All objects beyond this distance are in focus. When, therefore, the bellows of the camera is extended and the indicator points at 100, we speak of setting it at Universal Focus, or point of infinity.
Extending The Bellows For Focusing. By turning the lever or pressing the clamp directly beneath the lens at the base of the camera front, you release the lock, and the bellows can be drawn forward on the track or slide provided for this purpose. It is by sliding the bellows backward and forward, bringing the lens closer to or extending farther away from the ground-glass, that the focus is produced.
Scale Focusing. Observe on the left side of the camera a scale usually marked No. 6-10-15-25-50-100. These figures indicate the distance the camera should be placed from the object to be photographed. No. 6 would indicate that the camera must be six feet away from the object in order to have it in focus. The pointer being placed at No. 25 would indicate that when the camera is twenty-five feet from the object it would be in focus. It is safe when distances are over 50 feet to set the point of the indicator at 100, as this then becomes, practically, Universal Focus.
Ground-Glass Focusing. By focusing is meant, as stated above, the obtaining of good, clear outlines of the image, on the ground-glass, of any object being photographed. This focus is obtained by the racking, or drawing out of the bellows until the image appears perfectly sharp on all parts of the ground-glass. When the camera is used without a tripod, the focusing scale on the side of the camera bed is employed and the pointer is set on the line opposite the figures, indicating the distance between the camera and object, which should give a perfect focus. It is not advisable to alter the camera in any particular until thoroughly familiar with all its parts. To detect any error in the focusing scale, should your pictures be out of focus (i. e., not sharp), study the following instructions and learn how to correct the fault. Again, the proving of the focus is also a very good practice, for one cannot become too well acquainted with his instrument.