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.
Apparatus. Any camera that is suitable for ordinary photographic work, no matter what size, can be used to copy a photograph or any picture. For the amateur the only essential extra piece of apparatus required is a copying-board, or some kind of an arrangement for carrying the camera and the original to be copied, so that the plate and subject copied may be perfectly parallel.
518. Special cameras are made for copying and enlarging purposes, their bellows being of almost unlimited extension, but unless one desires to make a practice of this business in photography it is not necessary, as previously stated, to go to the expense of procuring any additional apparatus.
Lens. As objects which are to be copied are usually perfectly flat and contain lines, the lens employed should be one which will give a perfectly sharp, well-defined image on a flat plate, in which the straight lines of the original are rectilinear. It is, therefore, essential to have a lens which will give a flat field without distortion, and if this can be secured it is not necessary to take into consideration the rapidity of the lens. Everything else being equal, the shorter the focal length of the lens used for copying the better, because the bellows extension required will be much less than when using a long focus lens. To obtain a copy of the same size as the original, the distance between the lens and plate must be exactly twice the focal length of the lens, and it is very seldom the case that the average camera bellows will rack out far enough to allow the lens fixed to it to be used for this purpose. Hence, the advantage of a lens of shorter focal length. It is not a hard matter, however, to get over this difficulty, as will be seen from the following instruction.
520. As the single or view lens does not give a perfectly flat field, and bows out straight lines toward the edges of the plate, this particular kind of a lens cannot be employed when it is being used to cover a plate which is rather large as compared with the focal length of the lens. When compound or double lenses such as the rectilinear lens, are employed, nothing shorter than one of 6-inch focal length should be used on a 3 1/4 x 4 1/4 plate, or a 7-inch lens for a 4 x 5 plate, and the shortest advisable focal length to use with a 5 x 7 plate is 9 inches. If the modern anastigmat lens is employed, or one similarly corrected for the various effects of aberration, etc., it is possible to use an instrument of shorter focal length, as these lenses, due to their peculiar correction, are able to render a perfectly flat field on all portions of the circle of illumination. When using single and rectilinear lenses only the central part of the lens field should be employed, as the defects here are very small.
521. A good general rule, however, in selecting a lens for copying, is to employ one which is of practically the same focal length as the lens which was first used to make the picture being copied. It is possible then not only to make a reproduction exactly the same size as the original, but also to enlarge it to a considerable extent, if desired.
522. Since copying is simply photographing with a print as the subject instead of a person or object, the operation is not as difficult as it might at first appear to be. The securing of the desired size of the picture is, however, the most puzzling thing, with most beginners at least. Taking it for granted that you have a camera with a focusing-screen so that you can see on the ground-glass the image of the picture which you wish to copy, arrange the camera on the far end of the copying board. Upon looking at the image on the ground-glass after it is sharply focused, you will now observe that it presents a very tiny speck on the ground-glass due to the fact that the camera is at considerable distance from the subject.
523. An important principle of copying is here illustrated in that the nearer the camera is to the object the larger will be the image, and as in most copying it is desired to secure an image nearly, if not quite, as large as the original, the camera must be placed very near to the object. At the same time, the nearer it is to the object the farther must be the ground-glass from the lens, until if we are copying the same size, the lens is exactly twice its focal length from the subject, and the ground-glass is the same distance from the lens. If using a 4 x 5 camera equipped with a lens of 5-inch focus, the total distance from the ground-glass to the print being copied is 20 inches, and the lens will be midway between the two.
524. If it is desired to copy larger than the original, the camera must be still nearer and its bellows extension still greater. Here is where the difficulty usually comes for the amateur, as the average cameras, however suitable for most all other purposes, will not permit of bellows extension sufficient to do copying on any reasonable scale. To ascertain the limits in this direction, extend the camera bellows as far as it will go. Then place a foot-rule on the copying-board and push the camera towards it, keeping the camera square with the rule until the image of the rule is sharply focused. Focusing must be entirely done by moving the camera, keeping the bellows fully extended all the time.
525. If you are using a 4 x 5 camera, and the long way of the 4x5 plate-when the rule is focused-just shows a picture of 10 inches of the rule, it is clear that the greatest possibilities of this camera in the way of copying will be to give a picture of an object 10 inches long on a plate 5 inches long, i.e, half the size of the original, and to get that, the bellows must be extended to the utmost. If you possess a shorter focus lens, you will find that with the same extension of camera it is possible to get a sharp picture with the camera placed nearer to the object, and the image will be larger accordingly; therefore, one of the uses of a short focus lens is to give a large scale picture with a comparatively short extension of camera bellows.
526. But the average amateur does not care to buy a lens specially for occasional copying; moreover, it is not necessary. An ordinary magnifying-glass (the weaker the better) may be placed in front of the lens. A reading glass will do, but is rather more powerful for the purpose than is best. In either case, the glass should be a trifle larger than the lens employed so that the rim of the frame containing the magnifying-glass will slip over the flange of your regular lens. Should the magnifying-glass be too large, it may be centered before your regular lens by means of wooden wedges between the rim of the magnifying-glass and the lens-flange; or another simple way would be to attach around the lens barrel a light spring wire with three little prongs long enough to receive the rim of the magnifying-glass.
527. The effect of this extra glass is to make the lens apparently a shorter focus one, and to allow of the camera being placed nearer to the object being copied, and so secure a larger image on the ground-glass. As the new lens is not so carefully centered in its mounting as a photographic lens should be, and as it is not "achromatized " or "corrected " for photography, it will be found that the image perhaps will not be so sharp as can be obtained with the original lens by itself. But this does not matter very much, because in copying, the object being stationary, it will not move during exposure, so that by employing a small stop it is possible to counteract this want of sharpness and get as good definition when the supplementary magnifying-glass is used as it is possible to secure without it.
528. However, when one has not a reading-glass at hand, we would advise the purchase of one of the Ideal Enlarging Lenses manufactured by the Burke & James Company of Chicago. These are supplementary lenses that fit over the regular lens like a cap and are made in different sizes to fit any size lens you may have. Therefore, for the benefit of those who have a camera with a short bellows, we would advise that they provide themselves with a copying attachment. They are made in sizes from 1 5/16 to 3 inches in diameter and listed at from $1.50 to fit any size lens up to and including 1 1/2 inch diameter, to $3.50 for the largest size. The $1.50 size is suitable for any ordinary rectilinear lens 4x5 or 5x7. These auxiliary copying lenses, as you will note, are inexpensive and are made so that they will slip over the hood of the lens.
529. The Ideal Enlarging and Copying Lens is made to be used in combination with any photographic lens, increasing its power for copying to at least twice its original capacity. It will enable those possessing cameras, that are not long focus instruments, to photograph small articles to their full size or even larger, if desired. It will be found invaluable for copying pictures, making lantern slides by reduction with a short bellows hand camera and photographing small objects.
530. Some idea of the value of this lens can be learned when two photographs taken with and without the lens are compared as to size. A 4 x 5 photograph when copied with an ordinary camera will produce a picture about the size of a postage stamp, whereas when the Ideal Enlarging and Copying Lens is attached to the lens the photograph can be copied full size, or can be enlarged beyond its natural size.
531. This lens is composed of two elementary lenses of different density and refracting power, which are cemented together and form one corrected lens. The glass is of the finest optical quality of unusual hardness and brilliancy, and the curves are especially computed to give the best results. Each lens is accurately centered, and has ground and polished edges, which are points of great importance.
532. The lens is mounted in a brass cell, polished and nickel-plated, with adjustable springs, so that it fits over the hood of the regular lens like a cap.