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.
Overcoming Light Reflection In The Lens. It is always advisable to use a camera much larger than the plate you intend using for copying, because the farther the plate is away from the bellows the less danger there is of the plate becoming fogged near the edges by the reflection from the bellows. The reason for this is, that all of the light passing through the lens does not go to the formation of the image. A great deal of it falls upon the bellows and is reflected from there onto the plate. This defect is most noticeable when prolonged exposures are given, and on the portion of the plate where the deepest shadows appear in the picture you are copying. To overcome this trouble you should use a cardboard cone or hood fitted over the lens. The size of this cone should be as small as possible, but sufficient to prevent the light reflecting into the lens, and yet, at the same time, it must be large enough so as not to interfere with the angle of the lens. This hood or cone is also very useful when small pictures, such as miniatures or dark glazed objects which reflect more light, are being copied, as, in addition to other advantages, it prevents the lens mount being reflected from the glazed surface. The interior of the hood over the lens, of course, must be painted with a dead black surface.
Plates To Use. Ordinary slow plates are best for general copies. Very good originals are improved sometimes by using an Isochromatic plate, but old pictures that are yellow and stained should never be copied with any color sensitive plate (Isochromatic or Orthochromatic), as these plates tend to increase the blemishes much more strongly, being sensitive to the different colors that are found in old pictures. By the use of slow Orthochromatic plates you can sometimes make better copies from a fine print, one full of half-tones and color value, but never use them on stained, old, or faded yellow prints, for, as said before, these plates are sensitive to the different color values, and the defects will appear more prominent in a copy than they will in the original. It is advisable to use a slow plate, for the reason that slow plates have more latitude, both in the exposure and development. An ordinary slow plate, when over-exposed, can be handled much more easily in correcting the exposure during development. There is also less danger of over-exposing a slow plate. If under-exposed there is also more latitude in the slow plate while developing than there is in the fast plate. The slower the plate, the finer the grain of the emulsion.
The Use Of Orthochromatic Plates. In copying oil paintings or originals that are colored, you will find it necessary to use a color sensitive plate for good results. We will briefly describe the use to which Isochromatic and Orthochromatic plates should be applied.
476. The Orthochromatic plate is more sensitive to green and red; therefore, before deciding upon what plate to use when copying colored originals, note the color values you have to retain, and use a plate that will best preserve those colors. If you desire to copy a picture where green and red predominate, use the Orthochromatic plate. If, however, the predominating colors are green and yellow, use the Isochromatic plate. For the very finest results, when copying colored pictures, especially oil paintings, a color screen should be used in connection with a color sensitive plate. These screens are known as ray filters, and fit over the front of the regular lens. A very simple screen or filter, which we can highly recommend, is the Ideal Ray Filter, which can be used with any make of dry plate on the market. It gives true color value to all objects in a picture, whether landscapes, interiors, flowers, portraits, or to any picture with colors. It is well to bear in mind, when using this screen, that the exposure must be about three times that of the normal.
477. Another very fine filter is the lsochrom Absolute Ray Filter. This filter renders all colors in exact visual luminosity. The adjustment of this ray filter is quite perfect. It is put up in nickel plated cells to fit over the lens hood, and is supplied in a neat circular plush lined case, which can be used as a lens cap when required.
478. When purchasing one of these screens it is advisable to select the large size, so that you can adjust it to any size lens you may use for copying. The advantage of a filter or screen is as follows: While color-sensitive plates are effected by only a few distinct colors, yet by means of a suitable screen the plate is made sensitive to additional colors. There is no plate made which is as sensitive to green and red as to blue; so it is necessary that a screen be used-one that will subdue a certain portion of the blue. A yellow screen will do this, and very often, when the blue is quite brilliant, a deep orange or yellow screen is required to improve the results and equalize the colors, thus producing good values. These filter screens are sold or recommended by the plate makers, and should be procured with the plates when ordered.