Let us now consider the difference between the two types of lenses already mentioned.
This consists of one lens which is usually composed of two or more different kinds of glass. The ordinary single lens will only work at an aperture of about f11 or f16, and if of a medium or wide angle for the size of the plate used will render vertical or horizontal lines in the object which occur near the edge of the plate as curved lines. Although giving a very pure image, it is not fast enough for many kinds of work. I only refer to it because in discussing the doublet the use of the single combinations of the latter will have to be considered.
Doublet lenses may be what are known as ordinary Rapid Rectilinear or Aplanat; or of the most modern form known as Anastigmats. The former are composed of two similar combinations, and the latter, which are by far the finest type of all round lens now manufactured, vary considerably in construction. Either kind will render correct images without distortion, the R.R. generally being made to work at an aperture of f8 or sometimes f6, and the anastigmat from /6 or /4. It will be observed what a gain of rapidity we have if able to use a lens working at /6 or /4, and what an important help this is to the Nature Photographer. During the course of my own experience I have used a variety of lenses, and for the satisfaction of those who are not able to purchase the most expensive types, let me say many of the with 6 1/2-inch Busch Detective Aplanats, working of f6.5, and costing the very reasonable figure of thirty-five shillings each. These lenses are excellent value, and I feel certain would answer the requirements of those who contemplate taking up the study of Nature Photography. Another excellent lens at a low price is the " Aldis," made by Aldis Bros, of Birmingham. This works at f6 and is also of the modern anastigmatic type. There are some very good anastig-matic lenses sold by Messrs. Staley & Co., of Holborn, London, at prices considerably below those charged for the ordinary R.R. lenses a few years back.
Coming to the more expensive types of lenses, there are so many good ones at present on the market that the question of choice resolves itself more or less into a personal one; it is pretty much a case of " How happy could I be with either, were th'other dear charmer away." I have always had a leaning towards the lenses manufactured by Messrs. Taylor, Taylor & Hobson, Ltd., of Leicester, and in the " Cooke " lens they have produced a simple type possessing qualities which, at any rate in my opinion, it would be hard to beat. These may be had working at apertures of from /4.5 to /8. The brilliance of image and fine definition given by several different " Cooke " lenses I have used make me feel confident that any one who invests in one will not be disappointed. The exact and well-finished workmanship of the makers of these lenses is well known, and as their prices are considerably below those of many other types, this is an additional recommendation.
Another very fine lens is the Zeiss " Double Pro tar." This may be had composed of two similar combinations or two dissimilar ones. In the former case the full aperture of the complete lens is /6.3 and in the latter about /7 or /8. The great advantage of this type of lens is that each combination can be used separately, working at f12.5 and of about double the focus of the complete lens. If the lens is composed of dissimilar combinations then a choice of three different foci can be had, and three different-sized images can be obtained from the same standpoint, provided our camera has sufficient extension. On this account they are known as " convertible " lenses. While expensive, their performance is excellent, and they are very convenient in use and specially appeal to the advanced and experienced worker. Where the large aperture lenses of the anastigmatic type score is in the power they place in the hands of the photographer of working with very rapid shutters and in poor lights. On the other hand, they possess, in consequence of their rapidity, very little depth of focus at full aperture, and if much of this is required, they must be stopped down, and so lose some of their advantages.
If the beginner will be content to take the advice already given and commence by obtaining photographs of objects of a more or less stationary character, he will find that the ordinary R.R. lens when stopped down to, say, f16 or f22 will give him excellent results. As a proof of this may I draw attention to the illustrations numbered 3, 14, 22, 29, 37, and 40, which were all taken by the Busch lenses already mentioned. Allowance must, of course, be made for a certain amount of loss of definition in these reproductions, compared with the originals.
In recommending lenses or other apparatus by certain makers because I happen to have used the articles myself, I should like it to be understood I am perfectly aware of the fact that many other kinds on the market will, without the slightest doubt, produce equally good results. It is, however, impossible for one worker to experiment with, or use all, the numerous types of lenses on the market, for example, and the selection, therefore, becomes a matter of personal preference for one or other of the many good ones to be had.
Some workers use a Tele-photo lens. This is a form of lens which will give a magnified image of an object, and will therefore allow photographs to be obtained of shy animals or birds from a distance sufficiently great to prevent disturbing them. But there are certain drawbacks in its use. The exposures required, on account of the low intensity, are comparatively long ones, and the slightest vibration of the camera will cause a want of definition in the result. Certain states of the atmosphere also tend against obtaining good and clear results. With care, all these drawbacks may be overcome by a worker who has had some amount of practical experience, but as far as the beginner is concerned he will not be likely to feel that a Tele-photo lens would supply " a long-felt want".
In choosing a lens for Nature work preference should be given to one of fairly long focus, and it is a good plan to select the lens first and then see that the camera chosen will allow of sufficient extension to be able to copy an object full size. I should recommend for the 1/2-plate size a lens of between six and seven inches focus; for post card from seven to eight inches; and for 1/2-plate from nine to ten inches. If, however, one of the convertible anastigmats be chosen, the focus of the complete lens might be a little less than those given above, because the single combinations would supply lenses of longer foci, although working slower than the complete lens.
A lens is a delicate instrument to make and great care should be exercised to protect it from injury. When not in use it should be kept in a wooden or leather case lined with velvet, a cap being fitted over each combination. Never scrub the surface of the glasses, which have been very finely polished and might be seriously injured by hard rubbing. If the surface be dirty, use a very soft camel's hair brush, kept expressly for the purpose, to wipe it, or if this is not sufficient it may be gently wiped with a piece of clean old silk or very soft linen, in either case holding the lens with the surface being cleaned downwards, so that the particles of dust, etc., may fall off. This will help to prevent the surface being scratched.
In using, say, a six-inch focus lens to obtain an image life size, the camera must be racked out to a focus of twelve inches. If we are using an aperture of f6 this will become f12 under these circumstances, and will require four times the exposure necessary for the same lens and same stop, if used at about its equivalent focus of six inches, as it would be in photographing a distant object. This fact must be carefully borne in mind in using any exposure meter or tables of exposure where the size of stop used has to be taken into account.
A sky-shade to cut off some of the top light should always be used over the lens; this will enable one to obtain clearer and better results. It may be done by having a tube on the hood of the lens itself, or by arranging a black card to project from the camera front over the lens. Whichever of these two methods is adopted, it should be ascertained by the experiment of looking at the image on the focussing screen that the sky-shade does not project far enough to cut off any of the rays required to form the image.
To sum up: I cannot do better than give the usual advice, which is to get the best lens you can afford; but unless you thoroughly understand its capabilities, the price paid, whether high or low, will not govern the results obtained.