The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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lens
to have one instrument which is eminently adapted for all classes of work from portraiture to wide angle. Its large aper-ture, combined with its other special qualities, should make it useful in the developments of process work in which special, " shaped," or multiple diaphragm apertures are required. An-other new lens made by Mr. Dallmeyer - the Dallmeyer-Bergheim lens - is so corrected as to always give a soft focus ; and, more-over, its construction involves the principle of the telephoto-graphic lens, and the focal length is adjustable within certain limits. New lenses of the doublet type, in which astigmatism is very perfectly corrected, are the Voigtlander collinear lens, the Zeiss anastigmatic lenses, and the Goerz anastigmat, and a host of others based on similar principles ; many of these " new " anastig-
Fig. 99.
matics, being practically identical in form. The Cooke triplet is another recent form, but of a different character and construction. Lenses with an Element of Cylindricity. A note as to lenses with an adjustable element of cylindricity will be found on p. 274 of The Amateur Photographer for April 5th, 1901. Such lenses are sometimes useful in correcting distortion in the case of architectural subjects. (See Distortion, and also Anamorphote.) To enable the amateur to select a lens for his own use, several considerations are necessary, and although he may be to some extent guided by the vendor of the lens, the following may be of assistance: - The glass should be absolutely free from striae ; these can be detected by placing the eye at the focus of a lens before a strong light such as gas or a lamp; bubbles can also be seen, but too much stress
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