Exposure for copies is rather a difficult thing to determine owing to the variation of the object to be copied and the necessary camera extension. Of course, it is understood that as the distance from lens to plate increases, the f. value of a given stop decreases, and this rather has a tendency to complicate the process of mental arithmetic necessary to correctly figure exposures.
An f.8 stop is f.8 because its diameter is ⅛ of the focal length of the lens with which it is used. If such a lens is used for copying and the camera extension is twice the focal length of the lens, the stop value becomes f.16, which requires 4 times as much exposure as f 8.
For certain kinds of work the exposure may be fixed by increasing or decreasing the diameter of the stop with the increase or decrease of camera extension, the f. value of the stop in this way remaining the same.
For example, if an opening of f.22 is found satisfactory for the work at hand and the lens has a focal length of 16 inches, for each additional inch of camera extension the stop must be increased 1/16 of its original diameter.
This requires that the diaphragm ring be given a special marking, which is not always practical.Also the lens opening may become too large for the best definition.
The best plan for the photographer who has occasional copies to make, probably half a dozen at a time, is to use an artificial light that is practically uniform at all times. Select a stop that is small enough to give sharp definition on any subject it may be found necessary to copy.
When you have found the stop which will answer this purpose, measure its diameter very carefully so the same size opening may be used with any other lens you may choose to use for copying.
The next step is to make a rule or scale that may be used to measure any camera extension you may find necessary for producing a copy of the correct size. Spaces are marked off on this scale and numbered from one up, each space being exactly equal to the diameter of the stop you have chosen.To prove that your scale is right, if your lens has a focal length of 10¼ inches and the stop you have selected is f.8, the marking on your scale at 10¼ inches should be 8.
By using this same stop on all occasions it is only necessary to measure the camera extension with your improved scale to find the stop value for that particular extension.With artificial light of reasonably uniform intensity finding the correct exposure for one extension makes it possible to estimate the exposure for any other extension.
By C. E. Arnold Dallas, Texas
The stop values which theoretically double the exposure are of.4, 5.6, 8, 11, 16, 22, 32, 45, 64, 90,128,180, 256, 360, etc.
If your camera extension measures 11 on your scale, showing your stop has a value of f. 11 and your exposure is 10 seconds, the exposure at 8 would be 5 seconds, at 16, 20 seconds, at 22, 40 seconds, at 32, 80 seconds, etc., and these exposures would be practically correct and would permit you to estimate the exposures in between with a much fairer degree of accuracy than is possible by making copy exposures hit and miss.
Many photographers hate to make copies because of the uncertainty of exposure and the resulting loss in plates. A copying scale which will determine accurately this change in stop values and the necessary exposure for any camera extension, will simplify the process to such an extent that making a good copy negative will be a thing you can depend upon
There is good money in making copies if you make them right. The ordinary plate will answer if you have a good original. More often an orthochromatic plate and filter will give a better result.
Many photographs that are to be copied are slightly yellowed with age. An ordinary plate accentuates this fault, while an orthochromatic plate and filter will cut out the yellow and produce a negative almost as clean cut as the one from which the original print was made. Of the K filters (1, 2 and 3) the one which is equal to or darker than the discoloration of the print should be used. This can only be determined by experimenting.
If you have an important piece of work and can charge for your trouble it is often advisable to make the best negative possible - retouch the negative carefully - make an enlargement and work it up carefully and copy the enlargement to proper size for contact prints.
The more copies you make the easier you will make them, and you will find they will bring other business. The woman who brings you the poor little faded photograph is probably thinking of having other pictures made. Do your best to make a satisfactory copy and you will secure other business.
Time saved is as good as money earned.
FROM A SEED ORTHOCHROMATIC NEGATIVE
By C. E. Arnold Dallas, Texas
your family have been pleading with you for years - why not make that appointment to-day? Sitting for a portrait is a matter of minutes only - the same efficient methods you demand in your business are observed in ours.Your friends can buy anything you can give them - except your photograph,
THE PYRO STUDIO
FROM A BROMIDE PRINT
By Pearl Grace Loehr New York, N. Y.