In buying a stereoscopic camera, care should be taken that the lenses are accurately paired, and that the separa tion, 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 3-4- in.
It is better to have the lenses mounted on two sliding panels, to allow of the separation being varied between limits of 2 and 34 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 achro matic 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 //I 1, fj16, 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 61 in. focus may often be used with advantage in a stand camera.
Selection of Subject.—With the exception of copying from a plane surface, practically all ordinary subjects for photo graphy are within the scope of the stereographer. Land scape, seascape, architecture, interiors, still life, flowers, portraits, figure studies—all lend themselves to stereoscopic treatment. Landscape and seascape of the "open " variety, however, should be avoided : a strong foreground is essential to the making of a good stereograph, and the most pleasing effect is obtained when the points of interest are situated in several different planes, and form resting-places for the eye from foreground to distance.
Exposure and Development.—The good old rule of ex posing for the shadows should be the stereographer's constant guide. Detail is wanted in every part of the picture, especially in the foreground ; the exposure must therefore be ample. Under-exposure, always a bad fault, is trebly so in stereography, but a little over-exposure will not matter.
The great thing is to ensure good detail everywhere. The lenses should be stopped down sufficiently to give full depth of focus and sharpness ; no fuzziness or diffusion of focus is admissible. This rule should only be relaxed when circum stances demand an extremely rapid exposure, necessitating the use of a large aperture. In that case, special care must be taken that the actual subject of the photograph is in sharp focus.
Though exposure should be for the shadows, development should be for the high lights. Over-development must be avoided as carefully as under-exposure. Softness rather than pluckiness should characterise the stereoscopic negative, and the developer should be chosen with this in view. Whatever developer is used, it should contain only the minimum of bromide, and this should not be increased except in cases of known over-exposure.
Finishing the Aregative.—If by accident or miscalculation a negative of " contrasty " character is produced, reduction by persulphate of ammonium will usually effect a great improvement, unless the error is that of great under-exposure in which case destruction is better. It is futile to attempt to obtain a pleasing stereoscopic print from a negative that lacks detail.
There is no branch of photography in which cleanliness and careful manipulation are more important than in the production of the stereoscopic negative. Every defect is intensified in the stereoscope ; slight blemishes which would be unnoticed in an ordinary photograph are brought into startling prominence. The perfect negative should always be the aim of the stereoscopic worker ; there is no place in stereography for careless and slapdash methods. Pinholes and slight defects of that nature will occur, however, in spite of the best care ; and every negative should be minutely examined for them. Spotting must be done with extreme delicacy, or the remedy will prove worse than the disease. The density of the deposit in which the defect exists must be matched as exactly as possible, and the spotting medium must not be allowed to trespass beyond its limits. We find a No. o sable brush the best spotting instrument, and Indian ink, rubbed down in a few drops of water in which a little gum arabic has been dissolved, is a good spotting medium. To prevent the spreading of the medium, the tip of the brush only should be moistened with it.