Telephotography; An Elementary Treatise on the Construction, and Application of the Telephotographic Lens (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1899 Excerpt: ...screen at any distance we choose from the negative lens, and in order to find how many times we have magnified the image formed by the positive lens alone, we have seen that we must divide this distance by the focal length of the negative lens and add one. Calling M the magnification, and E the camera extension (distance between negative lens and screen), and ft, the focal length of the negative lens: M = j + 1 (I2) Rule.--To find the magnification: divide the camera extension by the focal length of the negative lens and add one. Conversely, Rule.--To find the camera extension necessary for a certain magnification: multiply the focal length of the negative lens by the magnification less one. E =/ (m-1) (13) These two rules apply for either near or distant objects. For a near object the focal length of the positive lens may be said to be temporarily increased and the image of a near object produced by it is larger than when the object is distant, obeying the law of conjugate foci. The conditions for the magnification of this image are not, however, interfered with. If the object is very distant (the position of the focus of the positive lens being that of its focal point) we can at once derive the focal length of the Telephotographic combination for any given magnification. It is simply =m/; F=m/1 (14) Rule.--To find the focal length of Telephotographic lens for any chosen extension of camera: multiply the focal length of the positive lens by the magnification. This may also be found in a perhaps simpler manner from the following consideration. As the positive lens fx is some multiple of the negative lens f2, if we call this m, then m =-j, and the focal length of the combination for a distant object, F = We +/i (15) Rule.--The focal length of a Teleph...