The Single Achromatic Lens

THE SINGLE ACHROMATIC LENS. • The single achromatic lens (Figs. 501 and 502) is always made of two, three, or even four parts cemented together, and is commonly of a meniscus form. The stop is generally placed in front, this being found to be the best position. These lenses are admirably adapted for land scape work, on which account they arel often known as landscape or view lenses ; having only two reflecting surfaces, they give remarkably brilliant images. The triple form (Fig. 502), now seldom met with, was at one time a favourite on account of its large angle of view ; but it was found to give an undesirable amount of distortion, and consequently fell into disuse. The single achromatic lens is commonly fitted to the less ex types of lenses intended for portraiture. There is a growing tendency to prefer certain modifications of the rectilinear and anastigmatic type to the Petzval, on ac count of their softer definition and greater depth of focus. A valuable feature in

some of the modern portrait lenses is pensive hand cameras, and, as its name indicates, is properly corrected for colour, thus allowing focussing in the ordinary manner. Its principal drawbacks, for all-round work, are its distortion and the fact that is not well adapted for use that, by a screw arrangement for slightly increasing the separation between the combinations, the amount of diffusion of focus may he altered at will; so that the photographer may secure either critical sharpness in one plane only or a more diffused definition over several planes. Lenses are also now made to work at much larger apertures than was at one with a larger aperture than f,11. In some of the better-class lenses of this descrip tion, however, distortion is so minimised as to be scarcely noticeable.