I will now try to illustrate the greater part of the principles laid down above by a single graphic example. You have, let us suppose, a 6-in. ordinary photographic lens and 2-in. " tele-negative " combined in a " tele-mount," " tube," or " setting," with a rack and pinion or other adjustment enabling the two elements to be suitably separated. You screw the whole telephoto lens into the front of your camera and rack out the latter until the tele-negative is 12 in. from the focussing screen. You then rack, or slide, or lever out the tele-mount, tube, or setting until a separation is reached which gives a sharp rendering of the centre of the object on the screen. Finally you stop the positive down to f/11. The result will be an image seven times as large [(12 in. ÷ 2 in.) + 1] as that given by the positive lens alone ; the focal length of the telephoto system will be 42 in. (6 in. X 7) ; and the aperture of the telephoto system will be f/77 (f/11 X 7).
When the facts comprised in the foregoing simple statement have been grasped the practice of telephotography presents very few real difficulties. Those which are most liable to occur are chiefly connected with illusions which amateurs sometimes persist in cherishing in spite of their almost obvious absurdity. You would be surprised if you knew how often I have to rub it into apparently intelligent people that the telephoto lens is an instrument with limitations, and that it is altogether unreasonable to expect that by spending four or five pounds on a tele-attachment you can readily produce results in every way equal to those which ordinarily would only be possible with a battery of lenses, the cost of which would soon run into three figures.
The three principal limitations to which I think the attention of amateurs should be specially drawn concern (1) speed, (2) definition, and (3) the circle of illumination or covering power. Before we go any further it is necessary to point out that in mentioning these limitations I am not alluding to the increasingly popular class of " fixed " telephoto lenses, of which the Busch " Bis-Telar," the Dallmeyer " Large Adon," the ' Ross Telecentric," and the Zeiss "Magnar" (the alphabetical order in this case removes every vestige of partiality !) are the outstanding examples. These are all true telephoto lenses, but each is adjusted to one unalterable focal length, the magnification being so small that a large aperture is retained, while the corrections are such that good, in some cases excellent, definition is secured without stopping down. Finally, the circle of illumination is in all cases adequate to the requirements which it is professed to meet. I need say little about these lenses here because they are normally used precisely as ordinary photographic lenses are, with the difference that with a short camera extension they give a relatively increased focal length. For portraiture and hand-camera work they are invaluable, and when a lens of this type is used, as in the N. & S. Reflex, interchangeably in the same shutter with an ordinary lens, a notable degree of convenience and efficiency is attained. Personally, I have found all the " fixed " lenses I have tried improved, as regards the brilliance of the results, by the addition of a longish hood, but, owing to the lowness of the magnification and the careful construction of the mounts, this precaution cannot be regarded as indispensable, and in a hand camera the extra projection is sometimes a nuisance.
Reverting to the aforesaid limitations, which have been so successfully removed in the case of "fixed' telephoto lenses, the matter of speed can readily be explained by what was said a little while back as to the aperture of telephoto systems. If the latter is the aperture to which the positive or ordinary lens is stopped multiplied by the magnifications, the " power " must necessarily be low in order to obtain anything like instantaneous results even in a good light. As a matter of fact, in summer and at the seaside, it is often possible to get very decent telephoto snap-shots, even with an ordinary positive stopped down a little, at four or five magnifications. But it is well for the beginner to realize at the outset that the telephoto lens, unless of the special " fixed " type, is essentially a slow lens, indeed a very slow one when a fairly high magnification is desired, and the necessary stopping-down has been effected.
As regards definition, again, the ordinary multi-focal telephoto combination formed by putting together an everyday photographic objective and a tele-negative lens in a mount or setting is apt to be disappointing to those who expect too much, as the ordinary amateur almost habitually does. It is all very well to say, " I have got a ' positive ' lens which I know to be of superlative quality, and I propose to procure a tele-negative which I am assured is as well corrected as it is possible for such a lens to be. Surely, the combination must be all right, and, if it isn't, then telephotography must be a queer sort of fraud." Alas ! it sometimes happens that positive and negative lenses actually by the same first-class maker do not give good definition over the whole of a plate, even with considerable stopping down. There are two explanations. First, all telephoto lenses necessarily work best at one particular magnification and consequent aperture, and those may not be the magnification and aperture at which you want to work. This is not, as a rule, a serious difficulty, as the majority of tele-negatives on the market are admirable compromises, working satisfactorily over a considerable range of magnifications. But there are also positive lenses which, while working superbly alone, seem liable to undue disturbance of some of their corrections when combined with negative lenses, unless the latter have been specially computed for use with them. I shall have something more to say on this point a little later, when I come to the question of choosing an outfit.
The third limitation noted above, that connected with the circle of illumination, is one of material importance where low magnifications are concerned. The simplest formula for ascertaining the circle of illumination given, irrespective of aperture, by a tele-photo lens is stated below. C is the diameter of the circle, f1 the focal length of the positive, f2 the focal length of the negative, M the magnification, and d2 the diameter of the negative lens.
Thus if the magnification is 6 times, the focal length of the positive 6 in., the focal length of the negative 2 in. and the diameter of the negative 1 in., the diameter of the circle of illumination will be
If you look into the above formula you will readily perceive that it is only possible to cover a fair-sized plate at low magnifications if your negative lens is one of comparatively large diameter. In the best tele-negatives the diameter is about half the focal length. If you have a 6-in. positive and want to cover a 5 X4 plate satisfactorily at 3 magnifications, you will find it necessary to use a 3-in. negative. But if you only have this one negative lens your range of magnifications will be small unless your camera extension is inordinately long. For at 10 magnifications a 3-in. negative demands a camera extension of 3 in. X (10 - 1) = 27 in. If, then, you have only one negative lens and a restricted camera extension you must be content with a certain number of magnifications, very few if you want to be able to work at X 3, and more if you are content with anything over, say, X 6.