it will be required ; but presuming that my readers are amateurs, and that many of them will be, or have to be, content with one lens, then a doublet, such as one of the newer anastigmatic in-struments, or a rapid rectilinear should be chosen. We have recommended the symmetrical doublet lens, or else the anastig-matic lens, for all-round work of the amateur as the most suitable lens, and it may be used for portraits, groups, architectural work, both exterior and interior, and landscapes. The next point to decide is the focal length of the lens ; this is usually fixed by the optician at a certain length, which increases with the size of plate for which the lens is required. The following may be assumed to be the focal lengths of the majority of commercial lenses for the given-sized plates : -
If more than one lens is required, the second lens may be a long-focus single or landscape lens of a focal length equal to times that of the doublet. If a third lens is required, trien a doublet of short focus may be chosen, the focus being about two-thirds that of the doublet. Thus, for a half-plate, we have first a rectilinear of 9-in. focus, then a single lens of . and
lastly a second doublet of 6-in. focus. Such a set of lenses will will enable us to obtain pictures of almost every character, and to obtain the images of objects varying in size. See Wide Angle. Many an amateur possessing a single or landscape lens, which he may have purchased at the outset with his outfit, yearns for a doublet, in the hope of improving his pictures, acting under the very general impression that far finer results are given by the latter lens. This, however, is a mistake. The sole superiority of a doublet consists in its working at a larger aperture, and in being free from distortion.
Testing a Lens. The first point to decide in testing a lens is its equivalent focus or focal length; the second point, the ratio aperture of the diaphragms ; the third, covering power of the