IN order to get a satisfactory photograph of any scene it is necessary that the exposure should be correct. The time of exposure required will, of course, depend upon the brightness of the image formed by the lens on the film, and this in turn will depend upon the aperture of the lens used and on the brightness of the scene photographed. The brightness of a lamp is measured in terms of its candle power; that is, a lamp is stated to be equivalent to 1, 5, 10, or 100 candles, the original unit being an actual candle, though nowadays the practical standards used are electric lamps which have been carefully measured and which are kept for use only as standards.
When the light of one candle falls upon an object at a distance of one foot, the brightness falling on the object is said to be one foot-candle. When we have 3 candles at a foot distance, the brightness would, of course, be 3 foot-candles, and if we use a 25 candle power lamp, the brightness will be 25 foot-candles. (Fig. 47). If we change the distance, the brightness will vary inversely as the square of the distance, because the cone of light which covers one square at a foot will embrace 4 squares of the same size at 2 feet, 9 squares at 3 feet, and so on; and since the same light that falls on one square at one foot is spread over 4 squares, at 2 feet distance, it is naturally 1/4 of the strength, so that a 25 candle power lamp at one foot distance gives a brightness of 25 foot-candles, and at 5 feet distance it gives only one-foot-candle brightness (Fig. 48).
The brightness of natural objects can be measured by means of a photometer, in which the brightness is matched with a lamp of known brightness. A convenient form of the instrument is shown in Fig. 49. In this, the scene is viewed through a hole in a piece of white paper, and the white paper, which must be backed on metal so that it is opaque, is illuminated by a small movable lamp of which the distance from the paper can be varied. In order to use the instrument, it is held up to the eye so that the brightness to be measured can be seen through the hole in the paper, and then the lamp is moved until the brightness on the paper is the same as that seen through the hole. Now, the brightness which the lamp throws on the paper can be calculated from the distance of the lamp, and consequently we can read on the instrument the brightness of the object to be measured.
Fig. 49. Photometer to Measure Relative Brightness.
In Figs. 50 and 51 are shown two landscapes which were photographed and at the same time were measured with the photometer, and it will be seen that the sky in these has a brightness of about 1500 foot-candles, while the deepest shadows in the foreground have a brightness of about 60 foot-candles.
It is often believed by photographers that the range of light intensities occurring in natural objects is very great, and that in an ordinary landscape, for instance, the sky will be enormously brighter than the shadows, but this idea is quite incorrect. In a bright landscape with heavy shadows, the sky is only about 30 times as bright as the deepest shadows, while in the case of open landscapes in which there are no close objects in the foreground, the range of intensities will be much less than this, the sky often being only 5 or 6 times as bright as the shadows. The range of light intensities, therefore, with which it is necessary to deal in ordinary photography will vary from, perhaps, 1 to 4 at the least up to 1 to 50 as a maximum, and the brightest part of a landscape - the sky - will have a brightness of from 1000 to 3000 foot-candles. This is the photometric brightness of the sky itself; but when we take a photograph, we are concerned not with the brightness outside but with that inside the camera; that is, with the brightness of the image which falls upon the film. This brightness depends upon the aperture of the lens, and we can calculate it from the fact that at an aperture of f.8 the photometric brightness of the image is about l/100th of the brightness of the object outside, so that the light from the sky falling upon the film will have a brightness of, at most, 30 foot-candles, and the shadows will be represented by a brightness of about one foot-candle in a photograph of a landscape having a brightness range of 30 to 1.
Now, let us consider how much time of exposure will be required for the film to reproduce the shadows, of which we see that the image formed on the film has a brightness of one foot-candle. In order to do this, we must know how much exposure is required by a film to make it developable. We can find this by exposing the film to a candle at a fixed distance and giving it a series of different times of exposures. It is convenient to have each of these exposures double the next one, so that one part is exposed for one second, the next for 2 seconds, the next for 4, and so on. If, now, we develop and fix the film, and then after it is dry find out how much silver per square inch we have produced in each exposed part we shall find that each time the exposure was doubled we added almost the same amount of silver (Fig. 52.)
It is rather hard to measure the amount of silver by actual analysis, but it can easily be done optically by measuring the blackness of the deposit, and this measurement of the blackness, which is proportional to the amount of silver per square inch, is called the "density". A density of unity is taken as the standard and represents the blackness of a deposit which lets through only l/10th of the light; it corresponds to a very small amount of silver - only about l/100th of a grain per square inch.