GENERAL PRINCIPLES OF TELEPHOTO LENS.
The telephoto lens is essentially a long focus lens of peculiar construction, and as it is fitted with an adjustment whereby the focus can be varied at will, it may fairly claim to be not merely one long focus lens, but a series of long focus lenses. The principles which govern the construction of the telephoto lens are by no means difficult to grasp. If a card with a pinhole be substituted for the lens in a camera, and the ground glass carefully focussed, a position will be found where the image pro jetted by the pinhole is a facsimile on the same scale as that produced by the lens, only, of course, not so sharp or so brilliantly illumin ated. It will be found that the position of the pinhole to produce this effect is in the plane occupied by the diaphragm of the ordinary doublet photographic lens : that is, in a plane corresponding to that occupied by the back optical node or projection centre of the lens. Now, although in the symmetrical form of lens the back equivalent or nodal plane is situated in the centre, or not far from the centre, of the lens, this is not by any means always the case; for in the primitive form of the uncorrected single lens its position will be found to vary with the curves of the surfaces. Thus, in a
double convex lens, the optical centre lies in the middle of the lens ; a plano-convex lens, on the front surface ; while in a meniscus lens it is entirely outside the lens itself. It there fore stands to reason that if a double-convex and a meniscus lens, both of equivalent focus, be attached to two cameras, both will produce pictures on the ground glass screen, in which every object is identical in size, but owing to the different position of the optical centre in the two lenses, the length of the camera ex tension will be greater for the double-convex than for the meniscus lens.