THE LENS.
THE lens, without doubt, is the most important part of the photographer's equipment, and it is essential that he should n F. v 1,0 .11.1.10G11 It 11-11 good one. He may con trive, with some per sonal inconvenience, to turn out satisfactory work with a faulty camera, but with an indifferent lens he can never hope to produce good pictures.
I do not intend to perplex the novice by attempting to lure him into the study of photo graphic optics ; yet it will be well for him to acquaint himself with the different forms of lenses in general use, and the special purposes for which they are constructed. I intend, therefore, to deal with the subject solely from a practical point of view, and to eschew theory entirely.
If certain expensive lenses of recent introduction are ex cepted, there are only three varieties of lenses in ordinary use with which the beginner need trouble himself, unless, indeed, he should aspire to portraiture, when another form of lens must be added. These consist of the single landscape lens, the rapid rectilinear, the wide-angle rectilinear, and, if portraits are to be taken, the portrait lens. I will now briefly describe the special pro perties of each.
The form of the single landscape lens is shown in fig. 5, which is a sectional drawing. The advantages of this lens are its cheapness, and its suitability for general landscape work ; its disadvantages being its slowness compared with the rapid rectilinear, and the property it possesses of causing the straight lines in a building to appear in the photograph slightly curved. Although the be ginner is frequently recom mended to select this lens for his first experiments in photography, I think the next lens which I shall describe, namely, the rapid rectilinear, is more suitable. This lens, the sectional view of which is given in fig. 4, is in more general use pro bably than any other. It is more rapid in action than the single lens, and can, therefore, be better used for taking portraits and groups, as well as for instantaneous views. It is a non-distorting lens, and will not produce the cur vature of straight lines which was referred to as being a fault inherent in the last-named lens. The third lens on our list—the wide-angle rectilinear—shown in fig. 6, is also a non-distorting lens, and its chief feature, as its name indicates, is the power which it possesses of including in the picture taken with it a very large angle of view. It is not so useful as the rapid rectilinear, both on this account and also because its rapidity is considerably less; but, where a second lens can be afforded, it is the one that should be chosen, as it is extremely useful for cer tain classes of work, notably the photographing of architecture, both interior and exterior, when it is often desirable to include a large portion of the subject in the picture. The portrait lens
(fig. 3), as its name implies, is intended for portrait work only.
On account of its construction, it is suitable only for that special work, and should not be used for any other, and as it is an ex pensive lens, when properly con structed by a maker of repute, the tyro will do well not to be tempted to purchase one unless he intends to make portraiture a special study.
Before we can intelligently further discuss the pros and cons of different forms of lenses, it will be necessary to define the meanings of certain technical terms to which it will be impossible to avoid reference.
The focus of a lens is the distance between the lens and the ground-glass when the image appears sharply defined thereon.
The stops or diaphragms are the plates of metal with which most lenses are fitted, which, when inserted in the slot in the lens mount provided for the purpose, contract or reduce the aperture of the lens. It will be easily under stood that the smaller the stop which is inserted, the slower will be the working capacity of the lens, and consequently the longer the exposure necessary. Instead of the per forated plates of metal, or Waterhouse diaphragms, just described, the lens may be fitted with what is known as the Iris diaphragm (fig. 7), which consists of a series of thin metal plates overlapping each other, arranged concen trically within the lens mount. By simply rotating a ring fixed to the outside of the lens mount, any sized opening may be obtained at will. This form of diaphragm possesses several advantages—being attached to, and forming part of the lens, it is not possible, as sometimes happens with loose stops, to either lose them or leave them at home. Most opticians now supply their lenses fitted with either form of diaphragm, and though the Iris diaphragm is rather more expensive than the Waterhouse phragm, I strongly advise the tyro to choose the mer. There is yet another kind of diaphragm to which reference has not been made. It is called the " rotating" diaphragm, and consists simply of a circular plate, in which different sized openings are cut. It is, like the Iris, permanently attached to the lens, and by simply causing it to revolve, any sized opening can be brought into the centre of the lens. It is generally only fitted to two classes of lenses, namely, the simple landscape lens and the wide-angle doublet.