THE SPEED OF EMULSIONS It is a fact that in their daily practice many photographers use developing formulas much stronger than what might be considered normal, i.e., stronger than would be required for purely normal exposures. The chapter on develop ment should be read (p. 120) before performing the following experiment as it is preferable that purely normal development be given.
To arrange for this test those who use plates or the film pack should make a cardboard cut off to shield three quarters of the plate and allow only one quarter to be exposed at a time. This is done as follows. Cut the cardboard just large enough to fit into the bellows folds as close as possible to the plate holder. Draw a straight line through the center of the card board both lengthwise and crosswise and cut out one quarter of it as thus indicated. With roll film a few whole exposures had best be devoted to the work as they can all be developed normally by the time method without cutting apart.
The subject to be photographed in this experi ment is arranged as follows. On top of a table and about 8 feet from a window which faces the sky place a box at least a foot square and over it arrange a white sheet so that on one side of the box it will face the camera, which should be placed about 2 feet to one side of the window and as near the wall as is convenient to work. The top sash of the window should be let down and the lower half of the window covered with a black cloth. The side of the sheet facing the lens should be smoothed so as to present an evenly lighted surface without folds or wrinkles so that its actinicity can be measured with the standard meter as already explained. Near this smooth part of the sheet should be placed some objects of various colors and textures, including if convenient, a medium gray, to aid in judging the correctness of the exposure and the gradation and latitude of the emulsion.
Now if a plate or a film pack is being tested place the cut-off in the camera close to the plate holder as explained, and with a large diaphragm in the lens, focus upon the sheet and other articles, confining the image of course to the quarter of the ground glass visible through the open corner of the cut-off. Should a roll film be tested point the camera by means of the finder and focus with the focusing scale or in the usual manner according to the construction of the camera used.
The camera and subject having been placed, there remains to establish the unit condition of actinicity and diaphragm and to make the test exposures. The experiment should be per formed on a clear day, as clouds, passing before the window, would vary the actinicity of the cloth. Now with the standard tinting medium
in the meter, expose to the white cloth and ascer tain the first appearance time of its light. Care should be taken not to shade the area which is being measured by holding the meter between it and the light. The meter may be held some what at an angle to the cloth so long as the smoothed part of the latter completely fills the f / 1 opening as has already been explained in the directions for using the meter (p. 38). Should the meter time prove to be 8 seconds as in the writer's present experiment, then as a trial cover about a of the window opening with a dark cloth or such portion of it as will reduce the meter time of the sheet to 64 seconds, thus securing a one actino intensity. Care should be taken that a 32 second exposure does not create the tint but that one of 64 seconds does so. When this condition is obtained the cloth may be said to have an actinicity of one actino.
Now on placing the unit diaphragm in the lens ( f / 64 or U. S. 256), which is the other nec essary constant condition, the test exposures may be made. After each exposure, when using the cut-off, remove the plate holder and turn the card to expose a different corner of the plate.
The speed of nearly all films happens to be 8 minutes or 512 seconds and taking this speed as a guide, the modern fast plate being consid erably faster than this and having less of the quality of endurance as to be explained, it may be judged that test exposures of 16, 8, 4, and 2 minutes will reveal a correctly exposed section. If however the plate being tested is known to be one of the fastest, then these exposures may be 4, 2, 1, and 1 minutes respectively. By normal development, the importance of which has already been emphasized, one of these sec tions will be seen to have had the correct ex posure and the time of exposure given to that particular section will be the "speed" of the emulsion tested.
It is clear that by such a method as that out lined the worker is enabled to select according to his individual taste or judgment, the speed for the emulsion. While one worker would select the exposure which gave plentiful detail and softness another might select that which tended to create a more decisive series of tone steps. One might say, for example, with re gard to a certain film, "I like best the effect which corresponds to an 8 minute speed in that film" while a fellow worker might class that film as having a speed of 4 minutes.