=General Considerations.=—The design and selection of lenses for aerial photography present on the whole no problems not already encountered in photography of the more familiar sort. Indeed, the lens problem in the airplane camera is in some particulars more simple than in the ground camera. For instance, there is no demand for depth of focus—all objects photographed are well beyond the usually assumed “infinity focus” of 2000 times the lens diameter. Such strictly scientific problems of design as pertain to aerial photographic lenses are ones of degree rather than of kind. Larger aperture, greater covering power, smaller distortion, more exquisite definition—these always will be in demand, and each progressive improvement will be reflected in advances in the art of aerial photography. But many lens designs perfected before the war were admirably suited, without any change at all, for aerial cameras. Of the utmost seriousness, however, with the Allies, was the problem of securing lenses of the desired types in sufficient numbers. The manufacture of the many varieties of optical glass essential to modern photographic lenses was almost exclusively a German industry, which had to be learned and inaugurated in Allied countries since 1914. In consequence of this entirely practical problem of quantity production without the glasses for which lens formulæ were at hand, some new lens designs were produced. Whether any of these possess merits which will lead them to be preferred over pre-war designs, when the latter can again be manufactured, remains to be seen.