This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1901 Excerpt: ...75; the larger numbers (from No. 1 upwards) have, however, a relative aperture of 1:25 and a field of about 85. The Protar-lens excels in anastigmatic flatness of field of a high order and is therefore particularly well adapted for wide-angle instantaneous photography outdoors under favourable conditions of light, also ...
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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1901 Excerpt: ...75; the larger numbers (from No. 1 upwards) have, however, a relative aperture of 1:25 and a field of about 85. The Protar-lens excels in anastigmatic flatness of field of a high order and is therefore particularly well adapted for wide-angle instantaneous photography outdoors under favourable conditions of light, also for landscape views and for large portraits and groups. Like all landscape lenses, the Protar-lens, when used singly, slightly distorts marginal lines. This defect is, however, of no consequence except with wide angles. For instance, the amount of distortion produced by Objective No. 3, having a focus of 285 mm (lil/4 in.), on a plate measuring 13X18 cm (5y8 X 7 in.) is scarcely noticeable, even in architectural views. The Double-Protar, composed of two single lenses, is as free from distortion as any of the best existing symmetrical lenses; neither does it matter whether the combination consists of two lenses of similar or of dissimilar focus. The combination of two lenses of similar focus possesses, however, the advantage, over those composed of two dissimilar foci, of having a larger relative aperture, hence greater rapidity. On the other hand, two dissimilar lenses provide a range of three foci, whereas two similar ones restrict the choice to two. An assortment of more than two Protar-lenses of different focus forms a Protar-Set, the elements of which can be used singly or in pairs. We supply any of these single Protar-lenses separately, having listed them under Series VII, with a view to facilitating the gradual acquisition of Double-Protars or of assorted sets of these lenses. (See "Protar-Sets"). Protar-lenses may be obtained without tube-mount if required to complete an existing set. In this ease prices are subject to a reda...
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