SELECTION OF LENSES.
The selection of the lens will probably give the beginner more trouble than any other part of the apparatus. Lenses should always be tested by an expert be fore purchasing ; mere examination is use less. Some methods of testing, which give however, the hand camera will do work as good as the stand camera ; in fact, many of the best. workers prefer a hand camera on account of its lightness and portability. Figs. 29 to 33 show a small selection of standard patterns, but hand cameras will be fully dealt with in a later section of this book.
a very fair idea of the value of a lens, are given on another page of this book. Such tests require some experience to carry them out properly, and the results ob tained should be verified by other means. A corrected lens, half-plate size, suitable for a beginner, can be purchased for from one to two guineas. Though periscope lenses are specially suitable for some work, the fact that they do not give a sharp photo graphic image makes them very unsuitable for general purposes. For all round
photography, rectilinear lenses are the best. They are not so rapid as portrait objectives, but are fast enough for most work, and are usually twice as quick as a single• lens. In the latter, straight lines occurring at the margins of the field of the lens are distorted either outwards or inWards, according as the stop is placed in front of or behind the lens (see Figs. 34 and 35). For outdoor work or for por traiture this distortion is of little conse quence, but for architecture, copying, and still greater, and the lens would be term a wide-angle lens. This is made clear Fig. 36, which is a picture taken with wide-angle lens. With a lens of norm angle the picture would have been lituite to that portion within the white lines Wide-angle is only a comparative ter applied to a lens when it is used to cove a plate the diagonal of which is greate than the focal length of the lens.