The advance of photography : its history and modern applications (1911)

130 THE ADVANCE OF PHOTOGRAPHY found to be a great advantage to use some yellow screen (see screens, p. 166). It is somewhat difficult at first to get accustomed to the use of such lens combinations, and the time of exposure usually is a great source of worry to the beginner. The necessary information is, as a rule, supplied with the better class of lens, and if one is careful to keep to the brands of plates which have been thoroughly mastered, little difficulty should be experienced with distant views ; the photography of near objects will perhaps require somewhat more practice. The camera extension, when such lenses are used, is often very considerable, and every precaution must be taken to avoid vibration, since, as the exposures are necessarily long, any vibration will detract from the value of the photograph. It is also necessary to take especial pain in focussing, if distinctness is aimed at, and with most lenses it is also advisable to use a rather small stop, and of course increase the length of the exposure proportionately. All photographs obtained by this means should be developed as slowly as possible, using a weak developer, and one which gives great density and clear shadows. Lenses have been constructed which can be used either as telephoto attachments to other lenses or by themselves. Such a lens is the Adon, which is composed of two achromatic combinations, the front being a positive lens of 4J" focal length, and the back a negative lens of focal length 2\". (In the more recent Adon these dimensions are slightly altered.) These are mounted in an aluminium setting, and can be separated very considerably. Pictures of very different sizes can therefore be taken by this lens, and it can be used with plates of various sizes up to 15 by 12". This lens gives good results when used for portraiture with a large aperture, and on account of its small size