We are now ready to insert in the camera the dark slide (duly loaded with plates film side outwards), and to withdraw the shutter. Dark slides containing plates should be protected from the direct rays of the sun, and it is a good rule to keep the focussing-cloth over the back of the camera and slide during exposure. Now the delicate question arises, for what length of time shall we expose the plate?
Ninety-nine out of every hundred photographers believe that they are born with a heaven-sent faculty of judging the time of exposure without the aid of meters or artificial calculations, or at least that they can correct any error in exposure during development. And we must admit that this faith is apparently justified in their works. We know of men with the experience of many years who are daily exposing several plates under all kinds of conditions - interiors, groups, portraits, and landscapes - and they invariably obtain results of technical excellence. The real truth is that modern dry plates permit of enormous latitude. There is a certain minimum exposure for each subject. Any period exceeding that minimum up to possibly thirty times will produce a negative in some degree representative of the subject. An example of this is shown on the page opposite. No. I of the two plates received 1/15 sec. exposure; No. 2 received I sec. in each case at f/16. They were taken on a roll of Ensign film, which was passed through a pyro developer for 4 minutes, and then fixed and finished off without any separation of the views. Had No. 1 been allowed to remain in the developer for two minutes, and No. 2 treated as an over-exposure with additional bromide, the difference between them would have been more difficult to gauge. As it is, No. 2 takes twice as long in printing as No. 1. The differences between the prints have almost disappeared in engraving.
Exposure 1/15 Second.
Exposure 1 Second.
But these casual methods of working are not satisfactory to the serious worker. It is necessary to know as nearly as possible the normal exposure for our particular view. We may vary it considerably, but we shall do so with deliberate motive, for it is in the variation of exposure that we can exercise more control than at any other stage over the ultimate result. Herein the artist can rise above the mere mechanic.
Normal exposure depends upon three principal factors:
(1) The lens and its diaphragm.
(2) The speed of the plate.
(3) The quality of light available.
And we might add a fourth, the distribution of high lights, half-tones, and shadows over the picture. For instance a picture consisting entirely of high lights, such as clouds, may only require about one-sixth the exposure given to an ordinary open landscape.
As a general rule, with each stage that the aperture of the lens is reduced the exposure must be doubled. The stops marked on most lenses are respectively f/6.5, f/8, f/11, f/16, f/22, f/32, f/44, and f/64. If we take 7/8 as our standard, then for f/11 the exposure must be double, and for f/16 fourfold. Some high-class lenses are naturally much more rapid in action than others of lower market value, but this difference is more apparent in dull weather than when the light is good.
Most makers stamp their plates with a number showing the actual degree of sensitivity of the batch, obtained by the Hurter and Driffield test. But the accuracy of the standard in comparing the speed value of various plates has been brought into question. Were the Hurter and Driffield method always applied with the same standard light by each maker, it would be enough to regard the speed number given as deciding this factor. In practice it is fairly correct. Most of the pocket exposure books provide a list brought up to date of the plates and films on the market, showing their actual speed as tested by independent means. A simple way of testing the speed value for our own purposes is to choose a view in which the features and lighting are well balanced, say a row of elm trees in the middle distance. The camera must be clamped tightly to a table or a stand rigid enough to remain firm while the dark slide is manipulated. The horizontal picture will be the one required.
On looking at the exposure table on p. 91 we find that the normal exposure given for the particular conditions of light is, say, 1/15 sec. Withdrawing the slide-shutter, we expose for half that time, or 1/30 sec.; then, replacing the shutter of the slide over one quarter of the plate, we expose again for another 1/30; pushing it in again to halfway, a third exposure is given, and then a final, with the result that we have secured on one plate a record of four exposures for 1/30,1/15,1/10, and about 1/8 sec. respectively. On developing these for a fixed time we shall be able to decide from which of these exposures we can obtain the relative factor for this brand of plates.
The chemical intensity of the light is, unlike the other factors, a variable one, and quite independent of the will of the operator. Conditions of atmosphere, geographical latitude, and the season of the year have to be considered, as well as the altitude of the sun above the horizon. The most intense light is present on a sunny, clear day for about an hour before noon; afterwards the evaporation caused by the heat of the sun intercepts a portion of the light. The lower the sun is in the heavens, the thicker and denser the layers of atmosphere through which his rays have to pass. Clouds and mist seriously decrease the amount of available light; dull, heavy clouds reduce it sometimes to about one-eighth that of a clear day at the same time of year.