This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Pairing Of Lenses. When two lenses are employed it is essential that they be accurately paired, both with regard to focal-length and quality. If it is found that one is more rapid in its action than the other, which is sometimes the case, this inequality may be balanced by using a smaller stop with the more rapid lens.
544. Where one set of lenses is employed for all your stereoscopic work, a normal-angle lens of from 4% to 5-inch focal-length should be selected. Distorted perspectives will be the result if wide-angle lenses are used for architectural subjects, the nearer objects being exaggerated in size in comparison with those in the distance. On the other hand, the stereoscopic effect is reduced considerably if a narrow-angle or long-focus lens is employed, for the objects near at hand are not included in the picture, and it is these near objects which assist very materially in producing the strongest stereoscopic effect. The narrow-angle or long-focus lens is suitable principally for long distance work, where it is impossible to work close to the object. By using lenses of a long focus - in other words, employing lenses intended for larger-sized work - you obtain a larger image on the same size plate.
545. For landscape work a lens having considerable depth of focus will be required. The lens which will give the best single picture, free from optical defects, is desirable for stereoscopic work. One point in favor of stereoscopic photography is, that pictures lacking in interest as a single photograph possess quite a charm when viewed stereo-scopically, and for this reason the beginner will often secure excellent results with inexpensive lenses.
Stereoscopic Photographs With One Lens. Although there are certain disadvantages which accompany the use of one lens, there are many ways of making the dissimilar pair of pictures with the ordinary hand camera having only one lens. If a 5 x 7 camera is used the lens may be mounted on a front board, which may be slid horizontally from one side to the other, between the two exposures. In this case a partition in the camera will be needed.
547. Another method is to use a 4 x 5 camera, making but one exposure on a plate. For this method a special tripod top should be employed.
548. A very simple form of stereoscopic tripod head is shown in Illustration No. 110. This is attached to the tripod at the center B, and the camera fastened in either notch A or C, and removed between exposures to the opposite end.
549. An improvement on the above apparatus, so far as the work of accurately shifting the camera between the two exposures is concerned, and also the effecting of the change with the least possible loss of time, is shown in Illustration No. 111. The camera is fixed on the block
Illustration No. 110 Stereoscopic Tripod Head-Simple Form See Paragraph 548
Illustration No. 111
Stereoscopic Tripod Head -
See Paragraph 549
A, with the lens pointing over B. When an exposure has been made the camera, which is on block A, is shifted to the left until the two blocks again touch each other, and then the second exposure is made on another plate. The parallel movement is regulated by a pair of links fastened to the two blocks. This attachment is fastened to the tripod head at the center, in exactly the same manner as the camera is ordinarily attached.
550. The single-lens camera, of course, has its drawback. For example, when making a picture of a street scene, or any view where there are moving objects, and where the exposure must naturally be rapid, the result produced is far from satisfactory. The objects that were moving would not have the same attitude or be in the same location in the second exposure as in the first. Where the twin lenses are used both exposures would be identically the same. Therefore, even for the amateur the single lens is not recommended for stereoscopic work, but may be used in case of emergency.
551. For those who expect to do much stereoscopic work, and yet do not want to invest in an expensive outfit. we would recommend the purchasing of a square-bellows 5x7 camera, made for use with twin lenses. The same camera can also be used with a single lens; all that will be required is an extra front board and an extra lens for 5x7 work.