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.
897. The slowness of the average achromatic lens and its uncorrected defect of curvature renders it of little use for interior portrait work. The principal requirement of an ideal portrait lens is, that the lens be one of long focus and work with a large aperture, thus giving greater speed. Its construction should be such as to enable it to give perfect definition over a limited field (18 degrees), at f.4 or U. S. No. 1, with complete coincidence of the chemical and visual foci. For large portrait work (plates over 5x7 inches), however, the portrait lens has not the advantages of the modern anastigmat, because its depth of focus at f.4 is so slight it is necessary to stop the lens down to a smaller aperture than is necessary when employing the anastigmat.
898. The principal type of portrait lens was invented by Petzval, in 1841, and was manufactured by Voigtländer. This lens remains at the present time the general principle of construction of all portrait lenses.
Dallmeyer Patent Portrait Lenses. The Dall-meyer Patent Portrait lenses are too well known to need a long introduction. Ever since they were introduced, in 1866, they have held their supremacy for studio portraiture. At the present time there is a strong tendency toward using anastigmat lenses for portrait work, but for studio portraiture, pure and simple, the Dallmeyer Portrait lens possesses an indefinable "something" which places them ahead of any other type. For outside work the anastigmat lens is excellent, and in many cases an absolute necessity, if the best results are to be produced.
900. The Dallmeyer Lenses are constructed on a different principle from the old or Petzval type of portrait lenses, and excel them in sharpness of definition, in freedom from distortion and flare, and in equality of illumination; while, in addition to this, they afford the means, by the simple turn of a screw, of obtaining greater equality or depth of definition. (See Illustration No. 82, Page 379.)
901. The construction of the lens is such that, with its cells all screwed home, it produces the sharpest possible picture of objects situated in one plane. Then, by unscrewing the barrel a turn, or parts of a turn, the previous intensely sharp definition becomes modified; i. e., the contrast of excessive sharpness in one plane, compared with want of sharpness in other planes, is balanced, producing the impression of a general distribution or depth of focus; and this in proportion to the amount of unscrewing. Nothing has been sacrificed in securing this new power, and it can be used or not, at the will of the operator, who will rapidly become proficient in judging what is necessary to produce any desired effect.
Illustration No. 83
Manner Of Diffusing. When it is desired to separate the lens combination in order to secure more definition, you should first unscrew the lens and then focus afterward. The separating of the individual lenses alters the focal point, and it is, therefore, essential that you accurately focus after the desired amount of diffusion has been secured. 903. Voigtländer's Portrait Euryscopes have been favorably known for their excellence, among photographers, for more than thirty years. Recently they have been improved by slightly modifying the calculations to conform to the peculiarities of the new Jena glass, from which they are now constructed, the most suitable grade of this glass being employed for the manufacture of the Euryscopes. The Portrait Euryscope, Series III., is distinguished for rapidity under the skylight and for artistic softness. It is intended only for heads and busts. (See Illustration No. 83.)
Illustration No. 84
904. Voigtländer's Heliar Lens. - This lens, which is suitable for portraiture, heads, busts, full lengths, and groups, is also made in a special mount suitable for high speed instantaneous work with the focal plane shutter for enlarging projections and telephoto work. This lens is the type of portrait anastigmats which are, to a great extent, taking the place of the old style portrait lenses. The Heliar lens is a carefully corrected high speed anastigmat lens with a medium angle of view. The difficult problem of obtaining high speed, without sacrificing optical perfection, is solved in this lens. The Heliar has a perfectly flat field and very-sharp definition. It possesses great brilliancy of image, on account of the entire absence of so-called "coma." "Coma" produces gray, flat images, while Heliar images sparkle with brilliancy. The combination of all these qualities explains the wide range of usefulness of the Heliar lens. (See Illustration No. 84.)
905. The usual trouble experienced with a portrait lens is that with full opening it will cut only heads and busts, but not full length figures and groups. For these it has not the necessary flat field - it requires stopping down, and this means loss of speed. The Heliar lens does not re-
Illustration No. 86 quire stopping down, except where more than usual depth is needed.
906. The lens consists of five glasses, comparatively thin, so as to transmit a maximum amount of light, a single glass being placed between two sets of two glasses securely cemented together. The body of the lens is largely made of aluminum, so as to reduce its weight. The iris diaphragm is placed between the middle and the rear lens, as will be seen on referring to Illustration No. 84.
907. To many professional portrait workers the introduction of the anastigmat type of lens, with its high optical corrections, has not appealed, on account of the sharpness of the image, which reproduces the human features and the texture of the skin with too great detail. This tends to hardness and gives an unpleasant effect, especially where the portrait is of comparatively small size.
908. Bausch & Lomb Portrait Lenses. - The portrait lenses manufactured by the Bausch & Lomb Optical Company are especially adapted to all around studio work. In Illustration No. 85 is shown the mounting for the regular Bausch & Lomb Portrait Lens, while in Illustration No. 86 is given a sectional view of the Bausch & Lomb-Zeiss Portrait Unar, which lens works at an aperture of f.4.5. It is a portrait objective giving any amount of softness or crisp-ness desired, and although an anastigmat lens, the diffusing attachment with which it is fitted enables one to secure excellent portrait effects without the harsh wiriness common to the average anastigmat lens. For full figures and group work this instrument is unexcelled, while for landscapes, interiors, copying and enlarging it is practically the equal of any lens on the market. Most lenses are better for some one purpose than for others, but with the Portrait Unar the whole range may be said to be covered equally well. It is quite true that this lens is somewhat expensive for the average worker, but where the results are taken into consideration it is money well invested.