Fig. 21 gives the general appearance of the. lens-mount. The centre portion is called the "Tube" and the end upon which the cap or shutter fits, the "Hood." The lens proper, Fig. 22, is the glass through which the image of any object in front of it passes to be received upon the focussing screen. The lens is fitted up inside the mount; it may consist of one or more pieces cemented together, giving to it the appearance of a solid piece. The image, after passing through the lens, is inverted. This is owing to refraction; this means that when the image reaches the outer surface of the lens it is twisted, and after passing through the substance of the glass it is again twisted and has imparted to it an upward or downward direction.

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Fig. 21.

There are two classes of lenses, namely, the "Single" and the "Double." The latter is the better one for architectural work, in order that all lines may be rendered true - see "Position of Diaphragm." They are again divided into two sub-classes: 1st, the "Round-field," and 2nd, the "Flat-field" lens. The flat-field lens is composed of glasses of different refractive powers, which compensate each other and give to the lens the power of being worked at a full aperture; that is, as wide open as possible with good definition all over the plate. Therefore it is a lens well adapted for use on a hand camera for instantaneous work. It is known under various names, as "Stigmatic," "Anastigmatic," etc. The possession of a good lens of this type places the handcamera worker in the best possible position for securing the finest results from snap-shot exposures.

The Lens Lenses Diaphragm Shutters /FirstStepsInPhotography 26

Fig. 22.

The Lens Lenses Diaphragm Shutters /FirstStepsInPhotography 27

Fig. 23.

The round-field lens gives an image which is sharply defined in some parts, but more or less blurred in others. This effect is owing to the lens having different refractive powers at different points of its surface. Fig. 23 shows this: the rays, A A, passing through the lens are brought to a focus on the principal axis at the point, A1; the rays, B B, come to a focus at B1, whilst the rays, CC, which approach the principal axis of the lens and become nearly parallel to it, are in focus at the point C1 much farther away.

The Diaphragm

This uneven definition is overcome by using a diaphragm or stop, to cut off the marginal rays, A A, giving a much clearer image and greater depth of focus; this, however, necessitates a much longer exposure. There are three kinds of diaphragms, Fig. 24, the "Iris," A, the "Waterhouse," B, and the "Rotating, or Wheel," C.

The Diaphragm /FirstStepsInPhotography 28

Fig. 24.

The "Iris" is the most frequently met with. It is made up of small metal plates passing one over the other, by which the opening or aperture is made larger or smaller. It is worked from the outside of the lens-mount by a milled-ring or a knob. Its position in Fig. 21 is shown by the milled band passing round the middle of the mount.

The "Waterhouse" Stops are small metal plates pierced with holes of varying size, showing a given relation to the focal length of the lens, which will be further described under "Apertures" Each stop is a separate piece, and is placed in position through a slit in the lens-mount at about the same position as the Iris diaphragm occupies.

The "Rotating" diaphragm is not often met with, except on cheap cameras. It is a circular metal disc pierced with circular holes of varying size, as shown in the illustration. It is screwed to the camera or in the lens-mount in such a manner that it will easily turn upon the screw, bringing the openings exactly in front of the lens as it is rotated.