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.
How To Preserve Lenses. Lenses should be kept in a pure, dry atmosphere, away from dust and damp. These conditions impair the perfect polish of a high-class instrument, and by scattering some of the light which passes through, produce a degree of fog in its images. Use an old, clean cambric handkerchief to remove dust. Never rub the glass, nor use whiting, leather, flannel, paper, or anything likely to contain a particle of grit; but brush lightly with such a smooth, soft duster as the before mentioned clean, old cambric handkerchief.
845. Hold the lens inverted and wipe the under side, that the dust may fall away from it.
846. A visible speck on the lens is of less importance than an invisible and general imperfection of polish, or a film of fine dust or moisture.
847. Lenses should not be left before a fire, nor in the sun to become unduly heated; neither should they be so cold, when used in a damp atmosphere, that moisture is condensed upon their surfaces.
848. In screwing together the parts of a lens, unless the screws are of Taylor, Taylor & Hobson's patent form, turn first in the wrong direction until the fittings snap together in a position for starting; then reverse the motion to screw them together.
Blacking For Wood Or Leather. An excellent blacking for wood or leather is made by mixing lamp-black with as much French polish as is needed to make it adhere sufficiently after being applied with a flat camel's-hair brush and allowed to dry. Too much polish will make the surface shiny, and too little will not secure the black pigment. The mixture may be made in a saucer and stirred with the brush, and thinned with a little wood alcohol, if necessary. It is well to try it first upon a piece of waste wood or card, and not to use it until the constituents have been so adjusted by trial. But it must be used quite freshly mixed. Purchased blacking is apt to be spoiled by long keeping.
How To Test Lenses. The image formed by a cheap lens is "dished," whereas that of a fine anastigmat should be as flat as the plates themselves. A simple but searching test can be made by any one who will pin a sheet of newspaper tight against the wall, focus, and expose a plate. To examine the ground-glass is not sufficient. A reliable test can be made only by exposing a plate. Care must be taken, however, to place the back and front of the camera accurately parallel with the surface to be copied, or the negative cannot be sharply defined throughout. The finer the lens the more sensitive it is to such error. A perfect anastigmat properly placed forms a perfectly flat image.
How To Focus. Most views contain objects at different distances from the camera, which cannot all be focused perfectly at once. One object gains in sharpness at the expense of another.
852. To secure the sharpest possible definition of the entire view is not, as some suppose, one of the most simple operations. The photographer who looks chiefly at the center of the screen, because it is easiest, and focuses to get sharpness there, lacks either proper interest in his work or proper knowledge of how to perform it. When he finds only the center of his photograph is sharp, and sees the deficiency elsewhere, he frequently exemplifies that old proverb: "A bad workman blames his tools." The wise photographer will, therefore, learn the secret of placing the ground-glass (in cameras where the ground-glass is moved instead of the lens-board) where it gives a just division of sharpness among the principal objects in view.
853. Requests are made daily for lenses described as "fast anastigmats to cut near and distant objects sharp simultaneously." "While light travels in straight lines, no such lens can do this fully, though you may be told otherwise by salesmen and by lens makers. Even the human eye makes no such attempt.
854. It should be clearly understood that depth of focus, zvith any given stop, is alike in all lenses of equal focal length, regardless of their construction. Depth can be increased only by using a smaller aperture, or by choosing a more suitable focal length.
855. The confusion partly arises because a lens of short focal length has more depth than one of long focus. At a given aperture the short one is better able to define the foreground and far distance, simultaneously, and so is often preferred for hand cameras, notwithstanding its giving a smaller image. For studio portraiture a lens of long focus is better, because it is used farther away from the entire subject. Better perspective is thus secured, and superior definition of the near and distant portions, which from a distance appear less separated.
856. The first thing to do in order to focus sharply is to understand the principles of a lens' action; to know what a good lens can do and what it cannot do; and to deepen that understanding by experience.
857. The second thing is to learn the secret of placing the ground-glass just where it should be.
858. "With a proper magnifier examine the ground-glass all over, not neglecting the corners, and find the important object which requires the shortest focus, and that requiring the longest. Note the position of the pinion-head or of the ground-glass screen when each of these objects is focused, and set the screen fairly between the two, favoring, as far as possible, the remainder of the view.
859. If only one object is to be focused, commence a series of rapid oscillations of the screen, passing each time the place of sharpest focus to equal blurring on either side. The amplitude of these movements must be gradually shortened until the screen comes to rest at the true focus. This operation may be repeated for confirmation, and under favorable circumstances the screen will come to rest within l-100th of an inch of the same place.