Before taking up telephotography - unless he is prepared to procure an entirely new and specially selected kit, the amateur will do well to make sure that his existing apparatus fulfils certain necessary conditions. In the first place, the lens which he is going to use as a tele-positive must be a really good one, preferably a high-class anastigmat working at not less than f/6.8. It should be fitted with at least one orthochromatic screen which should be a true " optical fiat," and it will be found a great convenience to have two such screens, one from X 4 to X 6, the other from X 10 to X 15. The camera must be one which is really firm at full extension, and it is imperative that it should have a thoroughly rigid front. There is no better camera for telephotography than the Sinclair " Una," because it has practically the rigidity of a " square-form " without being anything like so cumbrous, while the extension is quite adequate for any but extraordinary purposes. Messrs. Sinclair and Co. make a special " Una " with triple extension, but I prefer the ordinary pattern myself, especially in the 5X4 size, which I regard as ideal for all-round tourist telephotography. The full extension in this case is about 12 in., with which, by using a 1 1/8 in. negative lens, nearly 11 magnifications can be obtained after allowing for the projection of the tele-lens inside the camera. If the would-be telephotographer's camera is a " Una " the focussing screen is sure to be of very finely ground glass, but, if the grain be at all coarse, the screen should be changed for a micro-grain one. Frequently, too, it is advisable to procure a fresh stand for telephoto work, as one which does very well for ordinary photography may be quite useless when the camera is fully extended with a rather heavy tele-lens and a long hood projecting from it. Personally, I use my 5 X 4 Una on a stand intended for a 10 X 8 camera, and often, in addition, employ an auxiliary unipede lens support. Again, some operators carry only the flimsiest and " skimpiest " of focussing cloths for ordinary stand work. For telephotography one of good size and complete opacity is desirable.
Let us now take the essential new requirements, the tele-mount or setting, the tele-negative or negatives, the tele-hood, and the focussing magnifier. As regards the first, little need be said, as all makers of tele-negatives also supply mounts or settings, and as a general rule the beginner will procure his first tele-attachment - as the combined negative lens and mount are called - of one make. But care should be taken to let the first negative lens procured be of the greatest focal length which it is intended to employ, because if this is done shorter-focus negatives can be adapted to the same mount by having them mounted with extension pieces, whereas the converse is not always practicable.
Reverting to what I said on page 70 as to the importance of getting negatives which work well with existing positives, I regret that in only one case do I know of a series of up-to-date tele-negatives which have been expressly computed to work with a given series of ordinary or positive lenses. There may be several others, but I personally do not know of any negative lenses but those of Ross - in whose business I am sorry to say I have not a particle of interest - which entirely fulfil this condition. The Ross negative lenses, I happen to know, are specially made to work with the Homocentric, and, if you have one of the latter working at f/5.6 or f/6.3 - I cannot speak with certainty about the f/4.5 series - you may procure Ross negatives for it in perfect confidence that the results, through a long range of magnifications, will be as good as it is practically possible for them to be. In many cases a Ross negative works extremely well with positives of other makes, and frequently other positives and other negatives work together as well, to all intents and purposes, as if they were specially made for one another. But, unless they have been so made, there is no absolute certainty, even when the names on both positives and negatives arc of the highest repute. If, then, your positive does not happen to be a Homocentric, you must, in purchasing a tele-negative, to some extent take your chance unless the maker of the lens you have will guarantee to provide you with a negative or negatives giving at reasonable magnifications as compared with focal length really good definition all over the plate at, say, f/11. More than this it is hardly fair to stipulate for.
If you can only afford one tele-negative, the best ratio of focal length to the focal length of the positive is, I think, as 1 : 3. For a 6-in. positive a 2 1/4-in. negative is very suitable, and with a 5X4 Una this will give a range of magnifications, with the plate covered, from X 4 to rather over X 6. By adapting a 1 3/4 in. negative to the same mount over 7 magnifications can be obtained at full extension, and with a 1 1/8 in. negative nearly 11 magnifications.
A tele-hood is of serious importance, as without it the tele-lens is liable to internal reflections which seriously impair the brilliance of the image. Messrs. Sinclair & Co. have a very sound and efficacious arrangement consisting of two of their rectangular lens-hoods joined together, but personally I confess to a preference for telescoping tubes of aluminium, the smallest tube lined with dead-black cloth, paper or Atkinson's special " optical black."
Of focussing magnifiers there are several excellent commercial varieties, among which the beginner can choose for himself, giving preference to a pattern of fairly high power.