649. There are a great many photographers who possess, or who look forward to one day possessing, an anastigmat lens, who may not fully understand the special advantages of such an instrument. Thoughtful workers, however, may pause to ponder over the matter, and it is this class of worker that will benefit, to the greatest extent, by the following:

650. Not Always Desirable

Not Always Desirable. The modern lens is not always the most suitable instrument to use for all work. There are some subjects more pleasingly rendered with the blending of definition and diffusion, both of which qualities are characteristic of the spectacle lens or the single landscape lenses which have recently been introduced by the leading manufacturers.

651. Purely A Matter Of Speed

Purely A Matter Of Speed. The anastigmat is superior in point of speed, that is all. Take two lenses of 6 inches focal length - one of them an anastigmat, the other a rapid rectilinear. Select an ordinary landscape, with nothing nearer than 50 feet, and expose one plate with the anastigmat working at its full aperture - say, F. 6.3; then, changing the lens, expose the second plate, using the rectilinear at, say, stop F. 22, giving, of course, the proportionate exposure, approximately twelve times as long. On development of the two plates the negatives will appear the same, the one taken with the rectilinear lens being as crisp right up to the edges as the one produced by the anastigmat. We arrive, therefore, at this conclusion: That if we are working on subjects admitting of lengthy exposures, supporting the camera on a tripod and indifferent as to whether the wait while exposing on dark subjects is long or short; such as interiors, the rectilinear (and cheaper form of lens) will answer requirements. The anastigmat gives nothing more than the stopped down rectilinear, but it permits the same result (in this instance) in one-twelfth of the time.

652. The Difference In Definition

The Difference In Definition. A further experiment may be made by exposing two more plates on the same landscape, using the two lenses at the same aperture and giving equal exposures. As in all likelihood the largest stop on the rectilinear lens is F. 8, this must be employed, stopping the anastigmat to the same aperture. After development of the negatives make a careful comparison of the definition, and you will find that although the center of the negative made with the rectilinear lens will be very sharp, the margins will be less so. This will be particularly noticeable if such subjects as leafless twigs of trees come near the edges of the picture. We have already dismissed those cases where this blurring or softening of definition is wished for, and are assuming that sharp definition all over the plate is required.

653. Let us now consider why the anastigmat lens gives proper rendering of sharpness, while the rectilinear lens fails to accomplish this result unless its aperture is considerably reduced.