CHOICE Or A LENS.
The single lens is the cheapest, and a good single lens is superior to a poor rapid rectilinear. When only one lens can be had, choose the rapid rectilinear. If two, the rapid rectilinear and the wide angle. It is an advantage, where only the rapid rectilinear is possessed, for the two glasses or " elements " to be of different foci. For example, if one is of very long focus and the other medium, then medium and nar row angles are given when each is used alone, a wide angle being given when they are used together. It should be mentioned further that the rapidity of any lens de pends entirely upon its intensity ratio or the proportion which the diameter of the largest stop that will give the desired come foreshortened. They are a necessary evil, and should only be used in confined spaces for architectural work. The joss of light at the margins is also a serious de fect. The difference between the actions of wide and narrow-angle lenses may be gath ered from the illustrations in Figs. 46 and 47. From the foregoing it will be under stood that any lens is a wide-angle if used to cover a plate whose diagonal is longer than the lens focus. For ordinary work the focus should be equal to the diagonal of the plate. The necessary lengths for a medium angle or focus are : quarter-plate, 51 in. ; half-plate, 8 in. ; whole-plate, 11
in. ; 10 in. by 8 in., 13 in. ; 12 in. by 10 in., 16 in.
sharpness bears to its focal length. The larger this stop is the more light will pass, and the quicker will be the lens. A com mon fallacy is to suppose that a whole plate lens is necessarily quicker than a half- or quarter-plate. There is no advan tage in using the larger lens provided the smaller one covers with desired sharpness at the same aperture, and the use of the larger lens would cause more reflections in the camera, with possible loss of bril liancy in the picture. When buying a photographic lens, have it upon approval when possible, and then the following points should be tested for. It is a con venience to at once obtain some rough idea of the focus, and this may be done by focussing any distant object, measuring the distance from back of lens to plate in a single lens, or from diaphragm to plate in a doublet. The proportionate value of the stops when not marked is given approximately by dividing their diameter into the focal length. The fraction obtained is called the intensity ratio.