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.
Exposure. The necessary exposure required for copying pictures generally is difficult to state, for there are three principal factors that must be considered in making the exposure: First, the lens employed; second, the stop used; third, the color of the picture to be reproduced. With a lens of good aperture, requiring only slight stopping down, you can make a quicker exposure than with a lens of small aperture.
480. Another point to be considered is, that when stopping a lens down-if for instance we are making a picture the exact size of the original, and we use the lens stopped to f. 8 for ordinary work-using this same stop for copying work you actually stop the lens to f. 16, as the plate is then twice the distance from the lens, and will, therefore, require four times as long an exposure Ordinary copies from good originals, copied to the same size, will usually require from ten to thirty seconds, and if the original is to be enlarged, calculate on triple the exposure each time you double the size. For instance, if for a copy to the original size you require 20 seconds exposure, to double the size you would require 60 seconds, to triple the size you would require 180 seconds, etc.
Color Of Original. The color of the original makes a big difference in the exposure necessary. For instance, if the original is a good fresh print, with a medium-colored background, you will give less exposure than with a fresh print with a black background. Should the original be an old yellow print, discolored and stained with age, the color of the print must be taken into consideration. The best way to judge the necessary exposure is to note the appearance of the image on the ground-glass after stopping down sufficiently to give sharp definition, and then make the exposure accordingly. If any error is made in exposure let it be on the side of over-exposure, as the plate is very easily controlled in the developer. One can, after a little experience, train oneself to judge very accurately. In fact it is the only practical way of judging the proper exposure.
Latitude In Exposure. There is a wonderful amount of latitude in exposure for copies, as usually slow plates are used, and one should always expose for the densest portions and time them fully. It is in fact advisable to aim at over-exposure and then use a few drops of Bromide in the developer. As stated above, the exact exposure can only be ascertained by experience. After a few experiments with different kinds of copies one will soon be able to judge with sufficient accuracy to make every exposure correct.
483. For your first experiments a good way to ascertain the correct exposure is by a test plate. Select a fairly good original to copy from, and in order to arrive at the proper exposure for this kind of copies, note the appearance of the image on the ground-glass. If it is fairly clear and you judge the plate will require eight seconds exposure, place the plate-holder in the camera and cap the lens or close the shutter. Draw the slide about two inches and expose the first two inches say five seconds; then draw the slide one inch more and give another five seconds exposure, after which draw the slide another inch and give five seconds more exposure; then draw the slide entirely and give another five seconds exposure. You will then have four different exposures on this plate. The last portion will have had five seconds, the next portion ten seconds, a third 15 seconds, and the first exposure made will have received 20 seconds altogether.
484. Now develop the plate and you will probably find that you will have a correct exposure on some portion of the plate. You can then take another plate and give it the correct exposure. You should take your test plate and make a good strong proof from it, and on the back of the proof note the time of the different exposures, and any other data connected with the producing of the results. File this proof in your proof file for future reference. Experience, of course, will enable you to judge every exposure accurately by the appearance of the image on the ground-glass, and you will not need to experiment in this way.