The advance of photography : its history and modern applications (1911)

172 THE ADVANCE OF PHOTOGRAPHY Wratten Lantern plate has an average speed of 1 J Watkins, and the makers advise that the exposure behind a normal negative may be taken as 1000 candle-meter seconds divided by the Watkins speed of the plate. Thus 1000 divided by 1 J gives about 670, and this would be taken as meaning that at a distance of 1 metre from a 10-candle-power light, 67 seconds' exposure would be required. Films. — Lastly, for those who wish to carry about with them the means of obtaining a large number of photographs, and yet not be bothered with cumbersome parcels, the sensitive films, either in the flat or roll form will recommend themselves. Films can be now obtained with which the camera can be loaded in the daylight for exposure, and they can be taken out again in the daylight after being exposed. These films are rolled up with a strip of black paper in such a way that a good length of the black paper must be unrolled before the edge of the film is exposed. Again, some films are not continuous, but the sensitive film is made in sections to alternate with translucent paper. By this means it is possible to focus for the object before the exposure is made. Exposure Meters. — The most important step in the production of a good photograph is to decide upon the correct exposure to be given. It must be clearly understood that no care in the developing can make up for any deficiency in this respect. This point will be referred to in more detail later on. There is no necessity for amateurs who have barely a good working knowledge of the subject to make mistakes in this respect at the present day. Various kinds of exposure meters are upon the market at very reasonable prices, which indicate the correct exposure for plates of different speeds for certain lens apertures, the calculation being usually based upon the length of exposure