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.
The Lenses. The majority of photographers have only one set of stereoscopic lenses and must use these for the greatest possible variety of views. This makes the selection of a focal-length from 4 3/4 to 5 inches most desirable. As such lenses embrace only a relatively small angle of view on the finished picture they are not very desirable for interior views, for which a focal-length not exceeding 3 1/2 inches should be selected. It is, of course, of the greatest importance that a set of stereo lenses should be accurately matched; that is, that their focal-lengths be as nearly alike as possible - a difference not exceeding 1-250 of an inch, or l-10m/m, is allowable.
Testing The Matching Of Lenses. If a pair has not been matched by a reputable firm of lens manufacturers they can be tested by a comparison on distance and on close-by objects. If a pair matched properly on a distant view, it would still be possible that a difference in focal-length exists, but is compensated by a corresponding difference in the setting of the glass in the mounting; or, in other words, if we should have one lens of 4 3/4 inches focus and one of 4 11/16 inches, they may still show a sharp distant view at the same adjustment of the lens board when the shorter lens should also be set 1/16 inch deeper in its mounting. This will, however, be at once noticeable by focusing the same pair of lenses on an object only 2 or 3 feet away. The difference in sharpness will become visible and we can decide with certainty that such lenses do not match.
532. This rule works with equal accuracy in the opposite way, and may thus be used to determine if matched lenses are set equally deep in their mountings. Apparently they might fail to match and one would require to have its ring or flange sunk in the lens board in order to focus sharp with the other on distance. If after having done this the lenses will give equally sharp definition on nearby objects, they match optically and will be serviceable for stereoscopic purposes.
illustration No. 108
Stereoscopic Camera - Simple Form
See Paragraph 538
Illustration No. 105
Stereoscopic Reflex Camera
See Paragraph 529
Illustration No. 109
Stereoscopic Camera - Goerz Anschiitz Folding
See Paragraph 539
Copyright, 191, by H. C. White Co.
Illustration No. 106 Stereoscopic Views See Paragraph 535
Copyright 1901, by H. C White Co.
Illustration No. 107 Stereoscopic Views See Paragraph 535
Illustration No. 109a
Stereoscopic Camera - Kodak
See Paragraph 539
Illustration No. 112
Self-Transposing Printing Frame
See Paragraph 556 for portrait work. Larger openings reduce the depth of focus too much, and especially in stereoscopic photography-depth of focus is of the utmost importance for the final result. Soft definition destroys the plastic effect of the image - sharp, clean-cut negatives yield the best results.
534. For outdoor work it is often desirable to use lenses of longer focal-length than indicated for the all-around set of 4 3/4 and 5 inches. When the principal object is at a distance which precludes sufficient detail with this focus, longer lenses should be used, and better results will be obtained with a focus of 6 1/2 or 7 inches, under the condition, however, that some object in the nearer foreground be included in the view, as otherwise the effect of distance would be lessened and the picture made to appear flat.
535. In Illustrations Nos. 106 and 107 are presented stereoscopic pictures made with lenses of different focal-lengths. Fig. 1, Illustration No. 106, The Matterhorn, being a distant view, for the best results would require a 7-inch lens. Fig. 2, Pompeii, and Fig. 3, the Alexander Bridge, in Paris, are good examples of the use of normal-focus lenses for outdoor views, which is about 4% inches. Fig. 4, Illustration No. 107, Interior of the Mosque of el Aksa, in Jerusalem, is a good example of wide-angle interior views made with lenses of 3 1/2-inch focus. Fig. 5, The Grand Hall, in Palace Hotel, is an example of interior work made with normal-focus lenses, and shows the very restricted angle included in such views.