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Fig. 6.

The diagram, Fig. 7, shows a section of Mr. Dallmey-er'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 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.

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Fig. 7.

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. 3 represents the Dallmeyer Wide Angle Rectilinear Lens. It consists of two cemented combinations, 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 f/15; the position of the stop being nearer the back combination avoids the central spot or flare.

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Fig. 8.

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, etc. Being a double combination its work is more finished and round than that by the single lens.

Probably the two greatest rivals to the Dallmeyer Wide Angle Lens are the Actinic Doublet of Ross (Fig. 9), and the Steinheil Aplanatic (Fig. 10). The Ross Doublet consists of a crossed crown lens, cemented to its correcting flint lens, which is a crossed concave, the whole forming a deep meniscus the focus of which is equal to the back combination or about double that of the equivalent focus of the complete instrument, the posterior meniscus combination consists of a meniscus crown lens cemented to a concavo-convex flint lens. The two combinations are mounted rigidly with a rotating disk of diaphragms or stops placed midway between the lenses.

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Fig. 9.

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Fig. 10.

The Steinheil consists of a front and back corrected combination of precisely the same shape, mounted rigidly and having a rotating disk of apertures placed midway between the lenses.

These are three distinct types of view lenses of wide angle, and a comparison of the diagrams will show the wide difference in the construction of the two latter from the Dallmeyer; and while they stand unrivaled for the perfection and beauty of their work, which is about equal, the Dallmeyer obtains a great advantage over the others in the thinness of its lenses, which enables it to work with greater rapidity, and indeed it has been used with great success with the drop-shutter for photographing moving objects and other similar outdoor work.

While the possession of a wide angle lens is indispensable in the varied selection of out-door subjects, yet for many purposes it has been found they are unsuitable; and to meet this want Mr. Dallmeyer has constructed a modification of the wide angle, which he calls Rapid rectilinear. Its construction is shown in Fig. 11. The lenses of the front and back combination have the same general form as those of the wide angle, but they are of smaller diameter, being constructed for angles of 60 to 70 degrees only. It is four times as rapid, and is in fact an aplanaticand symmetrical lens, and may be regarded as the most perfect lens extant. As it admits of the use of a larger aperture it is well adapted for interiors, where there is space for its use, and for almost every purpose of out-door photography, requiring short exposure and no greater angle.

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Fig. 11.

The use of this lens has been greatly extended since the introduction of the rapid dry plate, which admits of its employment as a portrait lens in the studio, and in fact it has become a very popular instrument for portrait work, especially for the larger and life sizes, so that there is a strong probability that before many years the Rapid rectilinear and the D series of lenses of Dallmeyer will be the most useful and the most profitable lenses for portrait work in the studio that have ever been constructed, unless the near future shall give us something not now thought possible in optics.

Dallmeyer lenses are sold only by E. & H. T. Anthony & Co., who are the agents in this country, and as these lenses are quite expensive, and are indeed beyond the means of many, the Messrs. Anthony keep other and cheaper lenses, which are good of their kind, both for portrait and for out-door work, among which are the E A lenses for portraits, and the Platyscope and other lenses for views, etc.