The fidlowing working conditions may he adopted as a rough guide— Bromide papers c.p. lamp at 4 ft.
"Fransparencies, black tone Chloro-broinielt.. papers . 32 c.p. lamp at 18 in.
Chloride or gaslight papers .
c.p. lamp at 12 in.
Transparencies, warm tones • Although with practice it is not difficult to learn to count seconds, it is always preferable to rely on some mechanical time indicator. It ensures uniformity in prints taken from the same negative. The most practical indicator for this purpose is the metronome, adjusted to mark seconds or half-seconds.
553. Even the most experienced professional printer cannot avoid making errors from time to time in estimating the exposure for taking a print from a particular negative on a certain paper. Those who print only occasionally are very much more liable to make such mistakes, especially if they are using a paper with which they are not familiar. Rather than run the risk of spoiling several sheets of paper in succession, or of obtaining only inferior prints, it is infinitely preferable to sacrifice a small piece of paper for making tests. A piece of the same paper which is to be used for printing is used for making a series of exposures in steps, suitably adjusted to the negative.
A series of exposures so arranged that each shall be twice the preceding—so forming a geometrical progression in the ratio of 2-will allow the best time of exposure to be determined within a very long range, and by the use of the minimum of time and paper. It is, in fact, a general law in photography that exposures in a. constant ratio produce images in which the differ ences are practically equal. The number of test exposures to be made will obviously depend on the experience already acquired : the first esti mate is much more likely to be correct if the photographer has already ascertained the most satisfactory exposures for negatives which differ only slightly in character and if prints have been taken on the same paper under identical conditions of lighting.
The beginner should try, for example, ex posures of 4, 8, 16, 32, and 64 seconds. Within this range it is practically certain that the correct exposure will be found. A more experienced
worker can limit his tests to two exposures, 12 and 24 seconds, for example. It is not desir able, particularly at the start, to endeavour to economize paper in these tests. It is essential that in each of the test exposures it should be possible to examine satisfactorily the rendering of the lights and shadows, especially in the chief points of interest.
The printing frame, containing the negative and the piece of paper for the test, is covered with an opaque shield—a piece of card, thin sheet iron, etc.—and placed at the correct distance from the lamp, accurately adjusted. If, for example, it is desired to test the live times of exposure previously suggested, the shield, completely withdrawn for four seconds, will be placed so as to cover about one-fifth of the test-piece at the expiration of that time. Then the shield will be moved forward so as to cover an additional portion of the paper at successive intervals of 4, 8, 16, and 32 seconds.
The different portions of the paper will then have been exposed respectively for 4 4 sec.
4 + 4 — „ „ 4 4 + 8 + 10 --.- 32 HO 4 4 + 8 + 6 + 32 64 „ The test piece thus exposed should be devel oped at once under the conditions which will be described later for each variety of paper, the time of development and the temperature of the solution being very carefully noted.
The different parts of the test piece will show images increasing in depth ; that which best represents the tone values of the subject will be selected. Assuming that the development has been correctly carried out, if all the images are too light, all the exposures have been too short ; if, on the contrary, all are too dark,' all the test exposures have been too long. In either case the test ought to be repeated either by using a lamp of greater or lesser intensity, or by modifying the distance between the printing frame and the light, or by adopting a new series of test exposures. Should one of the partial images be a little too dark and another slightly too light, a fresh test is unnecessary ; an exposure intermediate between those corresponding with the two images respectively may be adopted.