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 Diffusing System. In the older types of portrait lenses the diffusing system is applied, but to adjust this system it has been necessary to remove the lens from the camera, or to remove the ground-glass, in order to get at the rear of the lens. Either of these methods consumes a considerable amount of time and does not permit of accurate adjustment of the amount of diffusion to the subject.
910. The later types of portrait lenses manufactured by the Bausch & Lomb Optical Company have a diffusing system, which can be operated by an adjusting knob which projects outside the lens barrel, on which a scale of values is engraved. In Illustration No. 87 is shown a sectional view through the Bausch & Lomb Portrait lens, giving an excellent idea of the optical and mechanical construction. The dotted lines show the position of the front lens of the diffusing system for greatest diffusion. This construction differs radically from that of other lenses, the front lens being movable instead of the rear one. By this improved construction any desired degree of diffusion of focus is obtained without disturbing the optical correction of the lens, as is the case of the rear lenses when the systems are movable.
Illustration No. 87.
Speed Of Portrait Lenses. The speed of portrait lenses of the same equivalent focal length varies directly with the diameter of the lens. It is, therefore, possible to preserve approximately the same focal lengths in portrait lenses, and obtain greater speed by simply increasing the diameter of the lens. This increase in diameter very materially increases the cost. The photographer will, however, find it very much to his advantage, especially considering the present tendency in photographic portraiture, to purchase as wide a lens as he can afford. It is always possible, by modifying the studio light or stopping down the light, to retard the exposure as much as it is desired when one has an extremely rapid lens. With a slow acting lens, however, it is quite impossible to obtain satisfactory exposures excepting with strong light, if one is to preserve the rapidly changing expressions of the countenance under normal conditions; or to secure well defined pictures of infants or children, especially in late afternoon work. The possession of a rapid lens adds several hours to the photographer's working day.
Illustration No. 82 Dallmeyer Portrait Lens See Paragraph No. 900.
Illustration No. 85 Mounting for Bausch and Lomb Portrait Lenses. See Paragraph No. 908.
Illustration No. 89 DISTORTED PERSPECTIVE. Made with Short Focus Lens, Distance, Lens to Figure, Eighteen Inches. See Paragraph No. 913
912. It is also possible to get good results in gray and cloudy weather. Rembrandt and other similar forms of lighting, especially the full shadow lightings, render it imperative to have a lens of the greatest speed, so that when the amount of light falling upon the features is reduced, the exposure may be brief.