(a) The Watkins Pinhole Lens is a cheap and desirable adjunct for those who wish to do this work with an ordinary camera.
(b) For Table of Exposures ; see Appendix.
Moreover, there is another peculiar phenomenon connected with our rays of white light. These rays are not simple rays, but are composed of various colours, and it is found that certain media which bend the rays of white light passing through them also resolve these rays of light into their component parts to a greater or less extent. So that not only have we to bend the rays of light as we require them to come to a focus on one plane surface, such as our sensitive plate, but we also have the difficult problem of keeping the rays intact so that the diverse elements forming the ray come into focus at that point.
The guiding of our ray of light is, therefore, a problem requiring not only the greatest mathematical precision in calculation, but the finest technical skill in lens manufacture. Some of the highest-class modern lenses consist of as many as eight lenses cemented in pairs of four each. Each surface has to be ground with the nicest accuracy so that it exactly fits the one in contact with it, made of glass of a different density, and when we think of the difficulties of manufacture we shall be surprised at the cheapness rather than the dearness of such a beautiful and precise piece of work. The standard of excellence has been constantly getting higher, and when to-day anyone talks of the superiority of old lenses, it is like the person who acclaims the " good old times," their goodness being a creature of the imagination. Buy the best lens you can afford, but the best is not necessarily the most expensive.
If you only want one lens for all-round work get one of the rapid type - preferably an anastigmat of some good make - and working at anything from f/6 or f/8, and of a focus somewhere near the measurement of the diagonal of the plate on which it is to be used. When the pocket permits we suggest either the Zeiss Series VII. Double Protar or the Ross Convertible Lens, in both of which the single elements forming the lenses are so perfectly corrected in themselves that they may be used for nearly every kind of work, and consequently we have two or three lenses of varying foci in one. Lenses of this nature are suitable for everything the photographer may want to do except very wide-angle work and very high-speed work. Other fine anastigmats are made by Busch, Dallmeyer, Goerz and Voigtlander, nor must we omit to mention the " Cooke," an excellent lens and not quite so expensive as some of the others.
THE SUPERIORITY OF MODERN LENSES.
THE BEST LENS FOR GENERAL WORK.
For work of great rapidity a lens with a larger aperture is essential, and in the first rank we would place the f/4.5 Zeiss Tessar and f/4.5 Ross Homocentric, both very desirable lenses for focal-plane cameras, and also for portrait work. The only objection to these lenses is the price, and to most photographers lenses of f/6 or f/8 aperture will suffice for every need.
THE BEST LENS FOR HIGH-SPEED WORK.
For the architectural photographer one other form of lens is undoubtedly required, particularly for work in confined situations, and such a lens is called a wide-angle lens, because it will cover a larger plate than is the case with the usual rapid types. As a general rule, wide-angle lenses are very small and the elements are so close together that light can pass through them at a very oblique angle. There is a good deal of misconception regarding wide-angle lenses by beginners. They imagine that a wide-angle lens is always a wide-angle lens, irrespective of the plate on which it is used, but such is not the case, it depends entirely on the relation of its focus to the size of plate used. For instance, a 5-inch wide-angle lens used on a 4 1/4 x 3 1/4 plate would not give any wider angle than any other lens of 5-inch focus used on the same plate, for the angle of view would be the same in both cases. Hut if we put our 5-inch wide-angle lens on an 8 1/2 x 6 1/2 plate we do get a wide angle, for the view which was illuminating the inside of the camera bellows when the lens was on a 1/4 plate camera is now covering effectively our larger plate. If we tried to put our ordinary 5-inch lens on an 8 1/2 x 61/2 plate we should probably find it was covering perhaps a 7-inch or 8-inch circle and that the corners of the plate were not illuminated, and consequently such a lens would be worthless as a wide-angle lens on a plate of that size. As a general rule, a lens which will cover effectively a plate, the shorter side of which is equal in length to the focus of the lens, may be termed a wide-angle lens. Editors who conduct photographic columns of papers are frequently asked whether a lens will be strained by putting it on a certain size camera. Now a lens is not like a muscle - no damage will accrue to the lens from its being placed on any size camera, for it will only work to the limit of its powers.