The fullest working aperture or opening varies in different lenses according to their construction. High-class, flat-field lenses frequently used on the best hand cameras have a working aperture of what is termed f/5.5 or f/6, but the greatest number of lenses have their fullest working aperture at f/8.

The meaning of the term f/8 is, that it shows the opening of the diaphragm to be equivalent to one-eighth part of the focal length of the lens; thus, supposing the opening to be one inch across, it might be understood to show that for all practical purposes the focal length of the lens is eight inches.

The diaphragm sizes generally met with are f/8, f/11, f/16, f/22, f/32, f/64. These figures are generally stamped upon the milled ring working the Iris, and on each stop of the Waterhouse.

The smaller the stop the longer will be the exposure necessary for the plate. This differs inversely to the square of the size of the stop or opening; thus, supposing f/8 required an exposure of one second, the stop, f/16, although only half the size, would require four seconds, but it will be found in practice that exposures at these large apertures are expressed in fractions of a second, and it is only when stopped down to f/32 or f/64 that the exposures will be counted in seconds; more, however, will be found under "The Exposure" on this subject.

The position of the diaphragm in relation to the lens in the single type has an effect upon straight lines in the subject when near the camera. When it is in front, as is usually the case, it gives to the lines a bulging shape, something like a barrel; if behind the lens, the lines would curve inwards. In the double lenses the diaphragm is between the front and back combinations, and therefore has a compensating action, so that the lines are rendered straight; hence the term "Rectilinear," by which name double lenses are sometimes known.

### Focal Length

The focal length of a lens may be roughly ascertained by pointing the camera at an object about 100 yards away, then carefully focussing. When the image becomes perfectly sharp upon the centre of the screen, the distance between the lens and the focussing screen in the single lens or the diaphragm and the screen in the double, will represent approximately the focal length of the lens.

### Angle Of The Lens

The focal length of a lens gives to that lens a given "Covering power" or "Field of view," that is, it will embrace a certain amount of the subject before it; this is called the "Angle of the Lens."

Lenses are spoken of as "Narrow," "Medium," and "Wide" angle. Fig. 25 graphically illustrates the working of these different angles of the lens on the same camera and upon the same object. The lines 1 and 2 might be taken to represent the boundaries of a road; 2 might be regarded as a blank wall, beyond which it is impossible to get to photograph the house on the opposite side; 3 is the point selected to place the camera to take the front of the house. The portions lying between A A might be considered as indicating the field of view of a narrow-angle lens; by replacing this with a mediumangle one, without moving the camera, it would be the matter included between B B, and by substituting a wide-angle lens, the whole between C C would be embraced.

Fig. 25.

Some double lenses are arranged to give three distinct lenses of different focal lengths. The lens in its entirety is of one focal length, the back lens after the removal of the front another, and the front without the back a third. It must always be borne in mind, however, that the diaphragm openings do not bear the same relation to each lens when separated, and the exposure must be made accordingly. Any double lens may be divided and used as a single; it will be found that in a great many instances the single combination is practically twice the focal length of the double. The focal lengths should be found and the relative values of the largest apertures ascertained after the manner described under "Focal Length." The focal length of a medium-angle lens - which is the one most serviceable for general purposes - should be equal to the diagonal measurement of the focussing screen.

The polished surfaces of lenses are very sensitive to any rough treatment, and the greatest care should always be exercised when handling them. To clean a lens, any superfluous dust should be removed by a camel-hair brush, and then a piece of soft chamois leather used to polish with. When not in use, lenses are best protected by enclosing in a proper lens case or a soft chamois-leather bag.

## Shutters

To shut out all active light from the interior of the camera - previous to making the exposure - the lens is provided either with a leather cap or a shutter. There are several forms of shutters. Those in common use are the "Roller-blind," the "Iris," and the "See-Saw."

### The "Roller-Blind" Shutter

The "ROLLER-BLIND" shutter, Fig. 26, is a shallow wooden box having circular holes front and back. Inside are two rollers - which are connected with the working parts of the outside - the bottom one is furnished with a spring; a piece of black opaque material - having an opening at its centre - is attached to the rollers. The shutter is set by pulling the tassel, A; this draws the blind up on the top roller, where it is held in position by the arm, B, acting on the projections on the cogs, C. The word "Set" is shown on the blind, and a click sound will indicate when it is set; it is liberated by pressing the bulb, D, which inflates the teat, E, causing the arm, B, to lift; the bottom spring acts immediately and pulls the blind down. The exposure is made as the opening in the blind passes the lens. The shutter must always be set before the slide of the plate-holder is drawn. The speed of the shutter is altered by means of the thumb-screw, F. This screw tightens the spring of the roller, the spindle of which passes through the shutter-box to the opposite side, where it is connected with a small dial, G, upon which the speeds are shown. An indicator passes round this dial and shows at what rate the blind, H, works. The speeds vary from 1/12th to 1/90th of a second. To reduce the speed from 1/90th to 1/12th after the spring has been wound up, the spring I is pressed to liberate F, the spring of the roller then runs down. For time exposures, the brass arm, B, is pushed over from "Inst." to "Time," then after the shutter is set, on pressing the bulb, the blind flies down until it is stopped by the large projection on C; the lens is uncovered and will remain so until the pressure is taken off the bulb, when it will close.

Fig. 26.