FOCUSING.
If your Kodak makes piures x inches or larger, you must look to your focusing as well as to the matter of light and stops. The manual accompanying the instrument explains how to do this, but some thing of the reasons for the necessity of it may be of value to you. It is our intention to make this book intensely praelical, yet a bit of theory often helps one in the accomplishment of pra6lical ends.
Suppose we wish to make a picture of an object six feet away ; the focusing point would be (with most lenses used in a 4 x 5 camera) seven inches from the lens. Now again, should we wish to make a piCture of an objea feet or more distant, we find that the focusing point will be six inches from the lens. So you can readily see that, with a camera of that size, it would be impossible to have a lens always set at the same point or distance from the sensitive surface, for if the nearby objects were in focus (sharp) the distant ones would be out of focus (not sharp), and vice versa, and for that reason we have what is called an " adjustable focus " ; that is, the front of the camera or that part which carries the lens is made so that it can be moved and the distance of the lens from the sensitive film or plate be increased or decreased.
The variation of focus is shown, greatly exaggerated, in the above diagram.
AArhen we say that a camera is " in focus " it is equivalent to saying that the sensitive surface of the film or plate, when in the camera, will come in contact with the rays of light reflected from subjea at this point or the point of focus.
On the Adjustable Focus Kodaks will be found a little scale marked with figures ranging usually from six to one hundred, and indicating feet. (They are also marked for meters.) You will also find a little pointer that, as you move the lens back and forth, will pass over the scale. Nov, should you wish to take a picture of an objeCel, say fifteen feet distant, you will move the pointer until it rests over the figure 15. This indicates that the lens has been brought to the proper distance from the focal plane (i. e., the surface of film or plate) for the focusing of obje6ts at a distance of fifteen feet. ObjeEts nearer by, or farther away, will not be absolutely sharp, this, however, depending largely upon the size of stop which you are using. See page 13.
While the adjustable scale for all cameras up to and including 5 x 7 inches will, when carefully used, give one just as sharp pictures as can be obtained by focusing on the ground glass, there is some satisfadion to the student of photography in focusing his camera on the glass before he sets out on his photographic career. It shows him what his lens is doing and why it impresses an image on the sensitive film.
When used with glass plates, the Screen Focus and the No. 3, 3A and 4 Folding Pocket Kodaks and the 4A Folding Kodak may be focused on the ground glass, there being a special Plate Adapter and Focusing Glass for this purpose. However, for the purposes of the study of this question
of focus, it is not necessary to purchase an adapter, as the back of the camera may simply be removed and a piece of ground glass laid against the rollers over which the film passes and the pidure may be focused on this. Be careful, however, to have the glass of such size that it may be placed in adtual contad with the rollers ; otherwise, it will not be in the proper focal plane and the lesson will be lost.
NOTE—The experiment of looking at the image on the ground glass may be similarly tried with the fixed focus Kodaks and will prove extremely interesting and instructive to the beginner.
To accomplish this, place the camera on a tripod, on the window sill or on a table. It will be better to focus on something outside of the house at first as the light will be stronger. Point the camera at some objed, a tree, for instance ; put the ground glass in position, use the largest stop and open the shutter. With your head about one foot away and your eyes on a level with the camera, take a piece of black or dark cloth and place over your head and back part of the camera, thus cutting off all light except that entering through the lens. Do not try to look through the ground glass, but at it. In a few seconds, or when your eyes get accustomed to the dark ness, you will see a pidure on its surface. The pidure or image you see will be just what you would get on the sensitive surface of your film or plate. You will, however, notice that the pidure is inverted, or upside down. This may excite your curiosity but will explain itself by aid of the following diagram : The rays of light from "A" passing in a straight line through " B " until they are interrupted by " C," on which they strike, form an inverted image of the object " A." Now move the lens back and forth until the tree is sharply defined on the ground glass. When the lens is at just the proper distance from the ground glass, the objed xvill be as " sharp as a needle" even if the largest stop be used. Now focus on objects at other distances. First, on an objed one hundred feet or more away and then on an object only eight feet away. You will notice that the farther away the object, the nearer to the focal plane (ground glass) the lens must be in order to give a sharp picture, and vice versa. And you will notice further, if you take the trouble to carefully measure the distances, that your ground glass tells the same tale as your focusing scale,—in other words, if you focus on an objeCt 25 feet distant and get it sharp, that the pointer on the focusing scale will point at, or nearly at, 25. You will also notice that all objeCts from 15 to 5o feet will be in good focus.