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.
Testing The Focus. In order to test the focus select a building or object in strong sunlight. Determine the distance this object or building is from the camera, which distance, in order to obtain good drawing, should be sufficient to allow the image not to appear crowded on the finder. Then, set the pointer on the camera to the corresponding number of feet on the scale attached to the camera bed, which should give a sharp focus.
76. After pressing the bulb the first time and opening the shutter, point the camera at the object upon which the sun is shining. Look on the ground-glass and carefully examine the image. If this image appears clear, sharp and distinct, the scale on the side of the camera is correct. If it is indistinct, and the distance from the object has not been misjudged, the scale is incorrect and cannot be relied upon. These scales, however, are usually correct, but in order to test them properly the camera must be placed upon something rigid, a tripod preferred.
77. Cover the head and camera with the focusing cloth. With the left hand gather the focusing cloth up under the chin. This will then exclude all light except that which comes through the lens and produces the image on the ground-glass. Look on the ground-glass. Do not try to look through it. It will take a little practice to enable the beginner to see the image clearly on the ground-glass. The image, of course, will be reversed (upside down). With the right hand reach to the front of the camera and extend the bellows by moving the front section containing the lens. Slide it backward and forward until the image appears perfectly distinct and sharp on the ground-glass.
Correcting The Focusing Scale. After securing proper focus, measure the distance from camera to object, and if this distance agrees with the figure indicated on the focusing scale, then the scale is correct. If they do not correspond, the scale can be corrected by focusing on some object 100 feet away. When a correct focus is secured at this given distance, and the actual distance does not register correctly with the pointer on the focusing scale, then remove the plate containing the scale and replace it so that the figure 100 is opposite the pointer. The rest of the scale will then be found to be all right. However, when possible, in order to insure perfect focus, it is advisable to focus on the ground-glass, paying no attention' to the scale. When using the camera without a tripod for snap-shot work, it is convenient, of course, to have the scale, and, therefore, it should be correct. As all reputable manufacturers test their instruments very carefully for this particular feature, it will be found that they are, in almost every instance, correct.
Finders. Most hand cameras and kodaks are fitted with a little box covered with a metal hood, a lens in the front of it and a small piece of glass on the top. This is called a finder, and is, in effect, a miniature camera. Everything visible on this finder will be visible on the ground-glass, and everything visible on the ground-glass should be visible on the plate when developed. Finders are fitted with fixed focus lenses, and, therefore, the object at any distance will naturally appear sharp on the finder.
80. If it is desired to secure the focus by the scale on the side of the camera, as, for instance, in kodaks, judge carefully the distance to the object to be photographed. With a little practice this can be accomplished successfully. A good plan is to measure by strides, and by pacing off the distance it will then be possible to judge more accurately the right number of feet. The better way, as said before, is to focus on the ground-glass. The finder should only be used when the instrument is employed as a hand camera; i.
e., without the tripod. Thus, the object to be photographed will be located in the finder instead of on the ground-glass. Remember, the finder is only used for locating the object, but not for focusing. The focusing must be done either on the ground-glass or with the scale.