The exposure is the most critical operation in photographic procedure, so much depends upon it. Its length is governed by four things.
1. Quality of light.
2. Class of subject ("Open" or "Shut-in").
3. Speed of plate.
4. Size of stop or diaphragm opening.
With a fairly correctly-exposed plate the operation of development is reduced to a minimum. Experience obtained by observation and practice is the best teacher.
The most active light is when the sun is shining, a blue sky, and some white clouds about. These conditions are slightly more vigorous after rain, the air being freed from dust. When the sun is shining, but no white clouds about, the light is not quite so vigorous. The reverse of the above is when there is a dull, heavy sky, or when storm-clouds obscure the sun. Then, again, the time of day and also of the year materially affects the time of exposure. The light gradually gains in strength from morning, reaching its maximum about 12 o'clock, after which it commences to weaken slowly until evening. From the early part of the year until about the middle its strength slowly increases, and then decreases until it gets to its weakest again, at the end of the year.
An "open" view, that is, one in which there are no objects in the immediate foreground to shut out much of the light, requires less exposure than one which is more or less "shut in." As an example of an "open" view, we might take a river scene with a stretch of open country; for a closed "shut-in" view, that in which a house, building or some such object occupies a position somewhat near the camera. Clouds and seascapes require short exposures.
The colour of the object to be portrayed also must be taken into consideration. A red-brick building would require longer exposure than would a whitewashed one. Interiors require considerably longer than exteriors, depending largely upon the amount of light admitted and colour of glass in window. The light in a room may be apparently as bright as outside; it must, however, be borne in mind that the glass only allows a certain amount of light to pass through it, retains some, and reflects the other, hence the difference in the activities.
Information respecting the speeds of plates will be found under "Sensitive Materials," "Degrees of Sensitiveness of Plates."
When the subject is focussed up at full aperture, and if the diaphragm opening is reduced, owing to the light being shut out, the focussing screen will appear much darker, and it naturally follows that when the light reaching the plate is reduced more exposure must be given.
As a guide for the beginner, it will be supposed that a slow-speed plate is being used and the lens stopped down to f/32, on a well-lit subject about midday in the brightest part of the year. The subject might be a house. In working upon such a subject, always try to include a bit of the side as well as the front - an exposure of three seconds might be given; if the exposure be made towards evening instead of midday, the increase in time must be two or three times as long. This will be a basis for the worker to begin upon; practice and observation will do the rest.
A little experimental work is most useful, For instance, if the worker will select a subject near home, and expose, say, three or four plates upon it, for the first count one second, for the second two, and so on; quickly exposing one plate after another, working with the camera in the same position and same stop in each exposure. After developing the plates in the same strength solution, using a fresh portion for each plate, there will be a marked difference in the resulting negatives; or to keep down expense, if working with a large-size plate, a row of houses may be selected. After focussing up and placing the plate-holder in position its slide may be drawn out for a quarter of its length, and one second exposure given; close the lens and draw the slide to half its length, expose for another second; repeat the operation for the third and fourth time. After development there will be four well-marked strips for comparison. Care must be taken not to move the camera in any way whilst making the exposures. Again, further experimental exposures should be made at different times of the day, and under varying conditions of light. Careful notes should be made for reference, and also the best negatives resulting kept as a type to work to. Much valuable knowledge will be gained in this way, and drive practical facts home in such a manner as it is impossible to do in any other.
It is well for the novice to accustom himself to form some idea of the activity of the light by the brightness or dullness of the image on the focussing screen. Attention to this will often help one to judge fairly accurately what length of exposure is necessary.
The exposures made when using a hand camera must be of short duration, being expressed in fractions of a second. The variation in the times of exposure is made by altering the speed of the shutter; if this be not possible, owing to the shutter having only one speed, then it is done by altering the size of the stop. In the very brightest weather good results are obtainable on plates of medium speed; at other times it is necessary to use the fastest.
Mechanical means, such as an "Exposure Meter," may be used to assist in estimating the strength of the light, and from it the length of the exposure that will be necessary. There are several kinds of exposure meters on the market.