TYPES OF LENSES Little need be added here to the note in the historical summary above on the subject of Portrait Lenses. It is a remarkable tribute to the genius of Petzval that the lines which he laid down in 1841 should be generally followed to-day in the lenses com monly used for portraiture, although naturally modifications and improvements have since been introduced by leading opticians. Ross, Dallmeyer, Taylor, Taylor and Hobson in Great Brit ain, and Voigtlander, particularly, on the Continent, have pro duced many portrait lenses of great excellence and, occasionally, of gargantuan proportions. In some modern studios, however, the portrait lens of old days is yielding place to the anastigmat, the wide aperture of which places it on a level with the Petzval construction in point of speed, while its more perfect corrections and covering power render it specially serviceable in the matter of studio groups. In simple portraiture, on the other hand, ex treme flatness of field is not always desirable, a round-fielded lens giving better "modelling." Since good portraiture has little use for "needle" sharpness of definition artist photographers since the days of Mrs. Julia Cameron have sought in various ways to modify the critical rendering of objects in the same plane by a well-corrected lens. One method of producing "soft" effects is by means of the diffusion created by partly unscrewing one lens of a combination. Latterly soft focus lenses have been produced by several opticians in which diffusion is secured by special compu tation, the objective being duly corrected for flatness of field, distortion and colour, but yielding an attractively soft image of marked pictorial quality. A third soft focus method is the employment of uncorrected lenses, as suggested by Bergheim, and carried out in the Dallmeyer-Bergheim lens (1895). Between 1903 and 1906 C. Puyo and L. de Puligny constructed several anachromatic systems for portrait and landscape work. More recently A. C. Banfield has experimented in the same direction with a view to keeping the aberrations in this form of lens within reasonable limits, the result being embodied in the Dallmeyer Banfield Portrait Astigmat of focal lengths varying from 18 to 3o in. and working with a maximum intensity of f/6.
The variety of lenses in use for field work is so great that only a brief mention of a few leading models is possible. In these the reputation of the makers for design and workmanship is fully maintained, but it is fair to say that in many less well-known pro ductions the requirements of all but specialists are adequately met, some important earlier patents having run out, and the sup ply of optical glass of high quality having been considerably ex panded by the entry of Messrs. Chance of Birmingham into this field of manufacture. For all-round outdoor work the converti ble cemented anastigmat is the most useful form, the performance of the single components in the case of doublets like the Zeiss "Protar," the Ross "Combinable," and the Watson "Holostigmat" being all that could be desired. With all these lenses asymmetrical as well as symmetrical combinations are practicable, thus giving a choice of three foci. In one series of the "Holostigmats" the single components can be used at the remarkably high aperture —for a single—of f/8.5. At the same time, the critical definition and increased rapidity possible with triple combinations like the "Tessars" make them extremely popular, their usefulness being enhanced, where desired, by supplementary lenses such as those mentioned above. Of triplets and asymmetrical doublets, with components one or other of which is uncemented or a single glass, a wide choice is available from the lists of Ross, Dallmeyer, Taylor, Taylor and Hobson, Wray and Aldis Brothers in Great Britain, and of Zeiss, Goerz, Voigtlander and others on the Con tinent, the apertures ranging from 1/2.2 to 1/6.3 and the focal lengths from two to three inches up to as many feet.