The rapidity of a lens depends mainly on its aperture and its focal length. Thus a lens of twelve inches focus will require four times the exposure of a six inch, with an equal sized diaphragm, and a quarter inch diaphragm will require four times the exposure of a half inch when used in the same lens.
The Photographic Society of Great Britain have recommended that the diaphragms of all lenses should bear such relation to the focal length that each should require exactly double the exposure of the next smaller. Now, if we turn again to Mr. Burton's table, we shall find that it is constructed on this principle, and that each stop is numbered so as to show its exposure. Obviously, the most sensible thing would be to get a set of stops made to correspond with this arrangement, but we will see how we can construct a table for stops of any size.
First, if possible, find the equivalent focus of your lens. If it is made by a known maker, you will find it in his price list, and if not, you may calculate it for yourself by the rules given in the various text books, provided you have a camera of pretty long focus. However, it will be near enough for our purpose if you get a sharp image of the sun on a piece of paper, and while you hold lens and paper, get some one to measure the distance from the paper to the diaphragm aperture, or, in the case of a single lens, to the center of the lens. Note down this focal length, and proceed to measure your diaphragms in sixteenths of an inch.
Then, with pen and paper, proceed to divide the diameter of each stop into the focus, and state the result as a fraction of the focus, thus f/8. For example, a Ross half plate rapid symmetrical has a focal length of 7½ in.; for convenience reduce this to sixteenths=120. A diaphragm measuring seven sixteenths will give the fraction f/17. Now let us see if any of these stops correspond with Mr. Burton's. The first two in his table will only be found in portrait lenses, but we shall probably find one to correspond with the third, if we are using a doublet lens; with a single lens we won't find any so large. Having picked out those that correspond, and filled in the exposure for them, we have now to deal with the odd sizes. Here is one, f/27, which is just half way between No. 16 and No. 32, but a moment's thought will show that as the exposure increases as the square of the diameter, it won't do to take the exposure half way between the two.
We have another factor to consider now: that is, the rapidity of the plate. If you use plates by a maker who has a name to sustain, you may be pretty confident that they are of fairly uniform rapidity, so after you have got into the way of working any particular brand, the best thing you can do is to stick to it. The exposures in our table are for plates of medium rapidity in good spring light. In my own experience I find that they just suit "thirty times" plates, or fifteen on the sensitometer; but then I like a full exposure with slow development, and I know that others find these exposures just right for "twenty times" plates developed in the usual way. The most rapid plates in the market will not be overdone with half the given exposures. It must always be borne in mind that an error of a fraction of a second in either direction may be corrected in development, and it is impossible to make a very serious error if you refer to the table.
We come now to the light. If you depend on the eye entirely in judging the quality of the light, it will sometimes play you tricks. The rays which are most active on the plates are those which have the least effect on the eye. We can, however, by chemical means arrive at an exact estimate of the active power, and for this purpose an actinometer is used. This is simply an arrangement whereby a piece of sensitized paper is exposed and allowed to darken to a standard tint, and by the time it takes to reach that tint the value of the light is judged. Capt. Abney has, however, pointed out that ordinary sensitized paper is not suitable for bromide plates, since there are conditions of light in which the plates will be fairly rapid while the paper will be very slow. He gives a formula for a bromide paper, which is treated with tannin in order to absorb the bromine set free during exposure, otherwise the darkening would be very slight. I used this paper for a while, but found it rather slow. The tannin also turned brown on keeping for a week or so. I then made some more, substituting for tannin potassium nitrite (not nitrate), which is colorless. This was an improvement, but still it was just slow enough.
However, noticing in Capt. Abney's article the statement that the bromide of silver should be as nearly as possible in the same state in the paper as in the plate, I thought "Why not Morgan's paper?" This, of course, is just bromide emulsion on paper, and if, as I suspect from its color, it contains a trace of iodide, why, so do most commercial plates. A sheet of this paper cut into strips, soaked for ten minutes in a fifteen-grain solution of potassium nitrite, and dried, gives a sensitive paper which darkens with great rapidity to a good deep tint, and keeps indefinitely. Here is some prepared last summer, which is still quite good. To use this paper make a little box so that a little roll of it can be stored in one end, and drawn forward as required beneath a piece of glass.
Bearing in mind that your table of exposures is calculated for the best spring light, go to the country some bright day next month with note-book, actinometer, and the necessary appliances for exposing a few plates. Select, say, an open landscape, and use your smallest stop. When all ready to expose, get out your actinometer and expose it to the reflected light of the sky for ten seconds (if the sun is shining, turn your back to it, and keep the actinometer in your own shadow); then put it in your pocket, expose a plate according to your table, and in case the light or plate should not be just in accordance with the conditions under which the table was prepared, expose other two plates, one a little less and one a little more than that first exposed. Then note down everything you have done—kind of view, stop, speed of plate, exposure of each plate, and length of exposure of actinometer.
When you get home, the first thing to do is to get hold of a paint box and paint the underside of the glass of your actinometer to match the darkened paper. Do this by gas light. Then scrape away a little of the paint, so as to let a strip of the paper be seen below it. After this develop your three plates with a developer of normal strength, and see which is best. If you have chosen a really bright spring day, and are using plates of medium rapidity, you will most likely find that exposed according to the table just about right.
Now let us see how we can use these aids in our field work. We have ascertained the correct exposure with a given stop on one class of view, with light of a given quality, but now suppose all these conditions altered. Let the view have heavy foliage coming close up to the camera, the stop be a size larger than that used in our first experiment, and the day rather dull. The table tells us what the exposure would be with this stop on this view, on a bright day; and if the actinometer take twenty seconds to reach the painted tint, then we must double the exposure given in the table.