TELEPHOTOORAPHIC LENS.--A specially constucted lens used in telephotography, q.v.
TELEPHOTOORAPHY.—The combination of the telescope with the photographic camera. The simplest form of telephoto lens is the ordinary opera glass. Mr. 0. G. Mason, in THE PHOTO GRAPHIC TIMES, describes a method of using this : He says : "An opera or field glass is a con venient and useful instrument in the kit of any touring photographer; and when he can easily and quickly attach it to his camera-box as an objective its great value is at once made appar ent. Mr. Lawrence's method of doing this at little cost is a good illustration of Yankee in genuity. It is not claimed that such a device will do all or as well as a telephotographic lens costing ten times as much; but it will do far more than most people could or would expect. Of course the field is quite limited, which, in fact, is the case with the most expensive tele photographic objective, and the sharpness of the image depends much upon the quality of the opera or field glass used. The accompanying views show the relative size and character of image by a forty-five dollar rapid rectilinear view lens and a four-dollar opera glass attached to the same camera and used at the same point.
The other illustrations show the camera as used and the method of opera glass attachment to the lens-board. It will be seen that the eye end of the opera glass is placed against the lens-board, one eye-piece in a slight depression around the hole through the center, and by a quarter turn the brace between the two barrels passes behind a projecting arm on a board, the focusing barrel resting in a slot in this arm, where it is firmly held in position by friction alone.
" As opera glasses are usually constructed for vision only, no attempt is made by the optician to make correction for securing coinci dence of foci of the visual and chemical rays of light as in the well-made photographic objective. Hence, it is often found that the actinic focus falls within, or is shorter than, the visual. When this is the case, the proper allowance is easily made after a few trials ".
Speaking of the telephotographic or large image lens, Dr. P. H Emerson, in his Naturalistic Photography, says: "This lens is constructed for photographing near objects of large size or distant objects of relatively large dimensions with comparatively short camera extensions. In all lenses that we have hitherto dealt with, the optical center of the combination has been contained within the lens mount. In the telephotographic lens now under considera tion, the entire system is positive in nature, form ing a real image ; its constituents are ordinarily a positive lens of which the focus is considerably longer than that of the negative component. It
is evident that if the components were placed in close proximity the focus of the system would be negative and hence no real image would be formed. By proper adjustment, however, of the separation between the two elements, a real im age can be formed at any camera extension chosen. The shorter the extension the greater is the amount of subject (always a narrow angle) included upon the plate, and the greater the ex tension the less the amount included upon the same plate.
"The greater the camera extension the longer is the corresponding equivalent focus.
" This lens, therefore, has a great selective power, and is invaluable in composing pictures from its selective capability.
" This lens differs in optical construction from ordinary positive systems, in that the optical center, or to be more precise, the nodal point from which the equivalent focus is meas ured lies outside and beyond the front com bination. The result of this is that the novice is astounded at the large size of image that can be obtained at a comparative short extension of camera.
" I am in the proud position, as it were, of being the godfather of this modern invention. In 1887 I asked Mr. Dallmeyer if such a lens could not be made, and he replied—no ! Next in the autumn of '96, I again requested him to assist me in the construction of an instrument for the dual purpose of obtaining large images of small objects in natural history observations, as well as for photographing distances with truer render ing as regards perspective, and in a manner that should obviate the depressed and dwarfed rendering given by all ordinary photographic methods, and suggested that the ordinary tele scope could be converted and arranged for such a purpose. This Mr. Dallmeyer very shortly ac complished, relying (as he has pointed out in a paper delivered before the Society of Arts) upon well known principles in the science of optics, and as the history of the subject has shown he was the first to do so in a serious manner for photographic purposes, and improvement after improve ment has followed the first design. Needless to say many others seeing the commercial value of such a construction followed in the wake, but we wish to place on record the true history of the origin of this valuable adjunct to the apparatus of the photographic world.