Photography in the Studio and in the Field / A Practical Manual Designed as a Companion Alike to the Professional and the Amateur Photographer

by Edward M. Estabrooke

Fig. 7.

The diagram, Fig. 7, shows a section of Mr. Dallmeyer's New Portrait Lens. There are two actinic combinations, of which the front resembles the Petzval lens; the back combination differs as regards the ratio of radii of the lenses used, the crown being a deep meniscus and the flint a deep concavo-convex, with their adjacent surfaces dissimilar; their positions also are reversed, the concavo-convex of flint occupies the external position, instead of as in the Petzval, and this lens being mounted in a cell capable of being unscrewed supplies the means of regulating the spherical aberration of the system at will. The lower portion of the diagram p94 exhibits a plan of the mount of the back flint glass lens; this cell admits of being unscrewed, one or more parts of revolutions of the screw indicated by an index and divisions; with this back lens screwed home this combination has all the good qualities of the old form of portrait lens, but with a flatter field and wider illumination.

There are three distinct classes of portrait lenses. The first are lenses of large diameter and aperture compared with their short focal length. In this class the greatest rapidity is obtained at the sacrifice of flatness of field. Of this class are the B and C lenses of Dallmeyer.

The second class are lenses of equal diameter and aperture with those just described; but with about double their focal length, and therefore less rapid, but with more field and wider illumination; of this class are the A series.

In the third class are long focus lenses, which, at three inches diameter have fifteen inches focal length; with the result of a larger and flatter field; they are, however, necessarily slow, but well calculated for out-door views, groups and copying. Of this class are the D series, which, since the introduction of the rapid dry plate, have become available for ordinary portrait work in the studio.

Fig. 8.

Fig. 8 represents the Dallmeyer Wide Angle Rectilinear Lens. It consists of two cemented combinations, p95 each composed of a deep meniscus crown and a deep concavo-convex flint glass lens; between the two, dividing the space in the proportion of their respective diameters, is placed a revolving diaphragm, the largest aperture of which is ^f₁₅; the position of the stop being nearer the back combination avoids the central spot or flare.

This lens embraces an angle of 90 to 100 degrees; it is quite free from distortion, and particularly adaptable for taking views in confined situations, such as interiors, views in narrow streets, &c. Being a double combination its work is more finished and round than that by the single lens.