When we look at an object or scene in the ordinary way, with both eyes, we obtain what seems to be a single impression of it; this, however, is really a combination of two dissimilar images formed by the right and left eyes respectively, as a simple experiment will demonstrate. Hold a book vertically at arm's length, with its back towards the eyes; close the left eye, and adjust the book so that nothing but its back is seen. Now open the left eye and close the right. You now see, not only the back of the book, but its left-hand cover also. Then, look at the book, in the same position, with both eyes : the view thus obtained will be seen to be a combination of the two single-eye images. The difference between these latter is due to the fact that the eyes view the object from points about 2 1/2 in. apart.
Our perception of the solidity of objects, and of their distances from us and from each other, is chiefly due to the fact that we see them at once from these two different view-points. If the reader will gaze at the objects around him with one eye and with both, he will perceive that when viewed with one eye only they lose much of their appearance of solidity, and that their relative positions and distances are by no means so readily estimated as when viewed with both eyes.
An ordinary photograph, taken as it is from a single view-point, is essentially a one-eye view, and cannot convey to the mind a perfectly true impression of the object or scene depicted. Stereoscopic photography, on the other hand, aims to reproduce the conditions of binocular vision, by means of two pictures taken from positions approximately the same distance apart as the human eyes. When these two pictures are combined in the stereoscope, the result is a representation of the scene in all its three dimensions - a representation so vivid in its realism that the beholder can scarcely believe he is looking at a mere photograph.
There is no extraordinary difficulty in stereoscopic photography. Any one who can take an ordinary photograph can take a stereograph, but to produce correct and pleasing results attention must be paid to certain important principles.
No special camera is necessary; any ordinary camera, of 1/4-plate or smaller size, can be utilised for stereoscopic work. An exposure is made in the usual way; the camera is then shifted horizontally to a position from 2 1/2 to 3 1/4 in. to the right or left, and a second exposure made on another plate. The movement from left to right or vice versa must be perfectly horizontal, and the two positions of the axis of the lens must be parallel. This may be ensured by the use of a light wooden tray (Fig. 59), with open back and fitted with a screw bush, B, for firm attachment to an ordinary camera tripod. The internal width of the tray from E to E should be 3 1/4 in. in excess of the width of the camera, the front of which must be kept in close contact with the fillet, FF, during both exposures.
It is obvious that this method is only practicable in photographing perfectly still objects, and the photographer who wishes to do general stereoscopic work will provide himself with a stereoscopic camera. This has two lenses of equal focus, mounted side by side, and an interior vertical division extending from the front (between the lenses) to the focussing screen. In other respects it does not differ from an ordinary camera, and may now be obtained in almost as great a variety of forms, from the simple box pattern at twenty-five or thirty shillings, to the high-class "reflex" at as many pounds. A very convenient form for stand work is the combined half-plate and stereoscopic camera, obtainable of several makers. For a hand camera, one taking the standard size of stereoscopic plate (6 3/4 x 3 1/4 in.) is preferable, on account of the reduction of bulk and weight. Many smaller stereoscopic hand cameras are now on the market, but for serious work the size last mentioned is none too large.
In buying a stereoscopic camera, care should be taken that the lenses are accurately paired, and that the separation, or distance between the lenses, is not excessive. The separation is determined to a certain degree by the size of plate to be used. In a camera taking plates of standard stereoscopic size, the separation, if fixed, should be about 3 1/8 in. It is better to have the lenses mounted on two sliding panels, to allow of the separation being varied between limits of 2 1/2 and 3 1/4 in., to suit the subject.
The considerations that govern the choice of lenses for ordinary photography apply equally to lenses for stereography. Excellent stereographs have been made with single achromatic lenses, but inasmuch as good definition over the whole picture is essential in stereoscopic work, and moving objects must be rendered with perfect sharpness, a pair of good anastigmats are worth their cost. Lenses of greater aperture than f/6 should not be chosen, however; the necessity for good depth of focus precludes the use of the wider apertures. The stops most used in stereography are f/11, f/16, and f/22, but when moving objects are being taken f/8 is useful, and for figure studies or rapidly moving objects f/6 is sometimes necessary. The most suitable length of focus for general work with plates of standard size is 5 in., but lenses of shorter focus are necessary in the case of the smaller cameras. On the other hand, lenses of 6 or 6 1/2 in. focus may often be used with advantage in a stand camera.