If we look at a dealer's catalogue we shall at first feel bewildered by the multitude of lenses all seemingly doing, or capable of, the same work, and only varying from each other by f values, or foci, or cost. The first thing that we notice is that lenses are marked with f values, and we must know the meaning of such terms as f/4, f/8, f/16, etc These numbers simply show the relation between the diameter of the cone of light passing through the lens and its focus. Very roughly with all doublet lenses we may take the distance from the diaphragm stop to the plate when distant objects are in focus and call it the focus of the lens. Then again, approximately the diameter of f/4 stop would be one-fourth of that distance, f/16 one-sixteenth of that distance, and so on in proportion.

In the case of a lens of 8-inch focus f/4 would pass a cone of rays 2 inches in diameter, f/8 one inch in diameter and f/16 a half-inch in diameter.* When, therefore, we see a lens marked in a catalogue as f/4 it means that the largest aperture of that lens is one-fourth the focus of the lens. As previously stated, this is approximately correct, and we have known some workers to have been seriously troubled at what they have imagined to be the wrong markings of the apertures, for as a matter of fact a diaphragm under some circumstances will pass a larger bundle of light rays than its own diameter. For those who want to accurately test the foci of their lenses and examine the effective apertures of the stops, particulars will be found in the appendix.

* For an explanation of the numbers on Kodaks, see Appendix.

THE LENS APERTURE, FOCUS AND THE DIAPHRAGM OR STOPS.

In purchasing a lens the work for which it is required must be borne in mind, and for general use lenses working at, say, f/6.3, are desirable ; for rapid work on moving objects such as express trains, diving, etc., we shall require an aperture of f/4.5, but for long distance landscape work an aperture of f/12.5 is amply sufficient. As a guide to the beginner we might mention useful foci for a complete outfit.

 For plates 4 1/4 x 3 1/4 in. 5 x 4 6 1/2 x 4 3/4 8 1/2 x 6 1/2 Lens for general use 5 in. to 6 in. 6 in. to 7 in. 8 in. to 9 in. 10 in. to 12 in. Lens for wide-angle work 3 in. to 4 in. 4 in. to 5 in. 5 in. to 6 in. 6 in. to 7 in. Lens for long-focus work 9 in. 1 1/2 in. 14 in. 16 in. to 19 in.

The smaller the aperture of the lens in proportion to its focal length the greater is the depth of field, i.e., the greater is the range between near and distant objects in focus at the same time, and the importance of this is often overlooked by those who purchase expensive lenses with large apertures, particularly for hand-camera work. With such lenses objects have to be most carefully focussed and there is little range for error, whereas lenses with small apertures permit of considerable latitude in focussing. A table is given in the appendix which shows how greatly the depth of field is extended, by stopping the lens down.

DEPTH OF FOCUS OR DEPTH OF FIELD.

At the commencement of this article we described the simplest form of camera, yet such a black box is severely limited in its scope. Unfortunately the tendency to-day is to try and cram into cameras all sorts of unnecessary movements which weaken the construction and are only required by one in a hundred, and then only for some special work.

Generally speaking, for all-round work a camera should have the following attributes : It should be well made and with a bellows as nearly square in front as possible.

It should have rackwork extension sufficient for lenses of foci given above.

It should have reversing back, revolving or otherwise.

It should have considerable rising front.

THE CAMERA.

It should have a securely clamped front, absolutely parallel to the surface of the plate, and one which does not sag or get out of parallel when extended.

A useful movement, but one not usually required when cameras have a great rise to the front, is either a swing back or, perhaps better still, a front which swings on the axis of the lens.

Some cameras fill the above requirements and are equally useful as hand or stand cameras, but for purely hand-camera work the reader would do well to read the article on Hand Cameras and their selection.

If the photographer wishes to do interior work, architecture, or telephotography, a good rigid stand is essential. The light metal stands, although exceedingly portable, may be useful for a certain amount of work with small hand cameras, but if hard wear and tear is probable then we should strongly recommend a tripod made of wood. In the case of telephotography an additional leg should be had for the front of the camera or to support the tele-photo lens.

Having the instrument for taking the pictures the next point for consideration is the medium on which to take them, and in the next chapter we shall consider the sensitive plate and film.

THE TRIPOD STAND.

The Falls Of The Zambesi.

Reproduced from negative taken with the Sinclair " Una " Camera.

E. G. Becher, Esq.