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.
Lens To Use. Any ordinary rectilinear lens will do, providing it is large enough to cover the print to be copied. A larger lens than sufficient to cover the original to be copied would require a longer bellows. Therefore, a short focus lens is preferable, as it does not require so much bellows length for reproducing a picture the same size as the original or even larger. The corrected lenses are, of course, the very best, yet, with care, good results can be obtained with an ordinary rectilinear lens.
Size Of Lens For General Use. For all prints up to, and including, cabinet size, there is no better general lens than the old style quarter-size Darlot lens, used generally for carte-de-visite pictures. This style of lens is of large aperture, thus giving good illumination, and there are, perhaps, more of these lenses used by professional photographers for their general copying than any other make. With a lens of this size, or any lens just large enough to cover nicely the print you are copying, you can make any size enlargement up to the largest size plate your camera will take.
Making Picture The Original Size. To make original size copies the distance between the lens and plate must be exactly twice the focal length of the lens. Say, for example, your lens is of five inches focus: Rack out the bellows ten inches; then by wheeling your camera close to the print to be copied you will find that when the front section of the lens is ten inches from the print to be copied you will have a sharp image on the ground-glass the exact size of the original. (See Paragraph No. 523.)
4G8. Stopping Down Lens. - When copying prints original size, do not diaphragm down any more than is necessary to secure a good clear definition. Too small a stop will make your picture wiry and harsh and will also copy more of the grain of the paper, which is objectionable in copies. When making copies larger than the original size, then small stops will be required in order to secure sufficient sharpness. First, it is always best to focus as sharp as you can without a stop, and then use as large a stop as possible and yet retain good, sharp definition.
Lighting The Original For Copying. The most satisfactory light to employ is daylight, as by means of a large flood of light you can illuminate more evenly the original to be copied, which is essential for the best results. Even with good daylight, in order to avoid grain in the negative you must exercise care when lighting your original, for if the light comes too much from one side or top it will accentuate the grain in the copy by making shadows from the rough surfaces which exist in the paper of the copy. So you must light as broadly and flatly as possible, and if the original is very rough and sort of pebbled, use a white reflecting screen on the side opposite the source of light. In this way, the shadows of the surface are filled with reflected light and they will not show badly, if at all. Under-exposure also helps to show the grain in the texture, lor the shadows, then, do not receive time enough and the high-lighis of the grain develop up strongly, thus accentuating the shadows. Moral: Always give full exposure.