It will be understood that in every case mentioned the same effect of bringing different planes into focus could be produced by the use of a small stop quite as well as by the use of the swing-back, the result being, however, the necessity of giving a much longer exposure. This may not be objectionable in the case of landscape sub jects, but it always is in the case of portraits. The swing-back is therefore particularly useful in portraiture, and specially so where this is conducted in an ordinary room where every possible saving in exposure is to he taken advantage of.
In the chapter on landscape work will be found reference made to the use of the swing-back for quite a different purpose from that which we have just described, and for one which must by no means be confounded with it. This is for the mere purpose of keeping the ground glass vertical when architectural subjects, which are on the whole above the level of the camera, are being treated.
Without our giving any detailed explanation of the matter, we may here state that if the ground glass of the camera is not vertical when a subject including vertical lines is included, these lines will not come out parallel in the resulting photograph, hut will converge or diverge at the top according as the camera has been tipped back wards or forwards. Accordingly, when the camera has to be tipped up so as to include the top of a building, the swing-back is used in the reverse direction to what is shown in the last cut. It is leaned forwards so as to make the ground glass again vertical instead of backwards. This will cause the vertical lines to be shown as parallel, I put will by no means enable a larger stop to be used than would otherwise be required. On the contrary, it will necessitate the use of a much smaller one. It will there fore be understood that the swing-back is an appliance not to use for the purpose just described until the lens has been raised as high as the movable front of the camera renders possible.
We now pass on to the subject of the length of ex posure which it is necessary to give to plates on various subjects.
We append—by permission of the author—a. table which was first constructed by Mr. W. K. Burton, and was published in the British Jounial of Photography.
We have altered one or two of the figures, somewhat reducing them, as we have found necessary, to agree with the exposures which we have been giving on Britannia plates.
A few words must be said as to the subjects which have been chosen as typical. It will be understood
that even with the same light every different subject requires a different exposure. It has been stated as a golden rule that we should "expose for the shadows, and let the high lights take care of themselves ;" and although the high lights do not always take particularly good care of themselves, at times going in for solariza Lion and such like vagaries, the rule is on the whole sound. It is the darker parts of our picture that must be considered in guiding us to the exposure which should be given.
A little consideration will show that all subjects can not come under any of the headings which we find given to the various columns of the table, but that many will come between the one and the other. Besides this, certain of these headings—such, for example, as " Fairly lighted interiors," and "Badly-lighted interiors "—are exceedingly vague. It is probable that the subjects chosen are as good typical cases as could readily be found, but they must only be taken as giving an idea of the exposure, nothing more.
We will take each column and say a few words on it. First, we have one headed "Apertures calculated on the standard system of the Photographic Society" (of Great Britain).
A committee of this Society decided some time ago to take as a standard, with which to compare other lenses, one whose aperture is one-fourth of its focal length, or which works at and to call this No. 1. A lens which 4 necessitated twice the exposure, or a stop which reduced the same lens to twice the slowness, had attached to it the figure 2. That which reduced it to four times the slowness was called 4, and so it went on-8, 16, 32, etc.
" Sea and sky " refers evidently to such marine sub jects as will make pictures without any further objects than the two mentioned, no boats or ships being intro duced, except perhaps in the distance.
" Open landscape" means that type of landscape sub ject in which there are no very dark shadows. We have such subjects often in river scenes, or, in fact, in almost any scene where there are no heavy shadows in the fore ground. It must be explained that a shadow in the dis tance never has to the photographic film the same or nearly the same darkness as a shadow in the foregroimd. The water vapour, dust, etc., which intervene between the distant shadow and the lens reflect a small amount of excessively actinic light.