On the principle of " safe bind, sure find," the more care that is exercised in securing an approximately correct exposure, the simpler and more satisfactory will all subsequent operations become. In ascertaining the correct exposure in any given instance, six points have to be considered:—(i) The speed of the plate. (2) The character of the subject. (3) The light. (4) The time of year. (5) The time of day. (6) The aperture of the lens.

For general purposes we may regard plates and films as being of four kinds, differing from each other in respect to their speed or light sensitiveness. These are:—Slow or ordinary, medium, extra rapid, and specially rapid; the three first-named being, roughly speaking, each twice as rapid as the one coming before it. The specially rapid is usually known under some fancy name, according to the maker, as for instance " Rocket" or something similar, and is, perhaps, half as rapid again as the extra rapid. Now, it stands to reason that what is a correct exposure for, say, " ordinary " plates will be too long for " medium," and still more so for " extra rapid." Next we have the character of the subject, and it should hardly need explanation to show that if one second be the right exposure for an open sea view in which there is no dark object, and which indeed is so bright as to make one's eyes ache, then it will not be sufficient for a woodland scene, which we value for its very dimness and cool shade. Remember that the plate in the camera is affected by the light reflected from the scene through the lens, and near objects reflect less than remote ones; hence distance is, generally speaking, lighter than objects near at hand. The sky is the lightest part of every view, whilst at the other end of the scale would be indoor scenes where the sky is intercepted by the roof, and light only admitted by windows and doors.

The next three factors—the light, the season, and the hour—may be considered together presently, and next we have the lens aperture.

Your lens, unless it be of a very cheap kind, is furnished with the means of altering the size of the opening, the chief purpose of which is to increase the defining power of the lens, a matter which does not concern us for the moment, and which will be better understood by reference to Mr. Welborne Piper's article on lenses in another part of the book ; but altering the aperture incidentally affects the amount of light which enters the lens, hence, the smaller the " stop " or aperture, the longer must the exposure be in order to admit more light, and the stops are so made that each aperture admits double the volume of light admitted by the next smaller, and consequently only requires half the exposure to be made. It may be explained that the stops are usually marked f8,f11,f16, and so on,* these signs meaning that the diameter of the aperture is 1/8 th, 1/11 th, or 1/16 th, etc, of f which stands for the focal length—that is, the distance between the plate and the lens when the scene is sharply focussed.

So now we have three factors towards calculating the exposure for any particular scene—the speed of the plate, the aperture of the lens, and the lightness or darkness of the scene itself. If you explain in general terms the nature of your subject and state the plate and lens aperture you propose to use, it might be expected that a person with a little experience would tell you about what exposure to give, and indeed a table of exposures can be made out for your guidance, such a one being given below but throughout the year and in every hour of each day the power of the daylight is changing, and this has to be taken into consideration. From midwinter to midsummer the light gradually increases and then as gradually declines in power, and so in each day from dawn to noon the light grows in intensity, and then falls off again towards sunset. In the Northern Hemisphere, then, midsummer at noon represents the maximum light power, and so tables of approximately correct exposures are calculated as for noon sunshine in midsummer, and must be increased proportionately as the time of day and year are removed therefrom. Such a table is here given, in which some typical classes of subjects are set down ; but if the subject does not appear to come within this classification a little independent judgment must be exercised, whilst if a smaller or larger stop is used multiplication or division by 2 of the exposures given for the next larger or smaller stop respectively must be made. The exposures below are given in seconds or fractions of seconds.

* To avoid confusion it may be said that in some cases for greater accuracy the stops are marked in decimal fractions, thus: f8.6, f 11.31, etc., or a different series of numbers are taken ; the relative exposures, however, remain the same, each larger stop requiring half the exposure of the next smaller, so that, the right exposure being found with any one stop, that with another is ascertained by multiplication or division.

Exposure Table

Exposure required to give Bright Crisp Negatives with the Developing Formula given on the Plate Boxes, In Bright Sunshine in June, mid-day.

Almost any text-book of photography will give you exact tables showing the alteration of the light at different hours and seasons ; but for the present purpose it may be sufficient to say double the exposure in March and April, and also in August and September, quadruple it from early November to end of January: and, as regards time of day, double the noon exposure at 9 to 10 a.m. and 2 to 3 p.m. increasing it to from four to six times when near to sunrise or sunset.

One more consideration remains, and that is the state of the weather. If the sun be lightly clouded over, double the exposure ; if dull and cloudy, multiply it by three ; or if very gloomy four or five times the normal exposure will not be too much.

Now there is nothing really complicated in all this. Our table gives us a clue to start with, and then we increase the time there given according to the season, hour, and weather.

But if any hesitation be felt, an actinometer, or exposure meter, can be resorted to, a little instrument which contains a light-sensitive strip of paper. This is placed in a shady part of the subject, and the time the paper takes to darken under the influence of the light is counted in seconds, and by then adjusting certain series of figures representing those factors which have been described above the required length of exposure is automatically given. There are many contrivances of this kind, including sliding scales to assist in calculating, but perhaps the best course is to work out the exposure yourself as above directed, and then seek by using an exposure meter to corroborate your own estimate. In this way, after a very little time, what may appear now as a laborious arithmetical exercise comes intuitively. Do not let the six factors and the columns of figures dismay you. Put F \t> stop in the lens, choose an open landscape subject, use a medium plate, refer to the table on page 13.

In the third column, sixth line from the top, you read 1/15 th of a second; but suppose it is in March, then double 1/15 th, which is, say, 1/8 second, and if it be afternoon 3 p.m. then double it again, which gives 1/4 second if the sun is shining, and if the sun is clouded double or triple the time—that is, from 1/2 a second to one second according to whether the sky be lightly clouded over or quite dull.

But the reader may have a question to ask as to which of the four speeds of plates to use. This must be largely decided by circumstances; but at the outset, or until he has learnt to get fairly accurate exposures, he will do best by keeping to the "happy medium," and except on some special occasions the medium plate will be fast enough for almost any purposes, and more easy to manipulate than a very rapid one, which should be regarded by the beginner rather in the light of an emergency plate. Moreover, by keeping to one speed of plate one factor in the calculation remains the same.

In very dull winter weather, however, an extra rapid plate will prove useful, compensating somewhat for the feeble light; whilst in bright summer days an "ordinary" or comparatively slow plate will be fast enough. And now, having exposed a plate, we proceed to develop the at present invisible or latent image.