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Figure 40 - Photographic camera

If the lens is convex, and the incident rays come from a point source, the incident wave front is spherical, and the emerging wave front is the surface of a sphere with its center at the image. If the lens is concave, the curvature of the wave front is increased in passing through the lens, and the image is behind the lens.

The Photographic Camera. - The simplest application of lenses for optical purposes is in an ordinary photographic camera (Figure 40). A lens or combination of lenses in one end of the camera produces a real image of an external object on the photographic plate or film at the other end of the camera. The distance between the lens and the photographic plate or film can be altered so as to focus the image on the plate. Sometimes the photographic plate is replaced by a ground-glass screen on which the image is cast, and then the lens is moved into such a position that the image is sharply focused on the ground glass. The plate or film is then substituted for the ground glass. The bellows В serves to exclude all the light except that which comes from the object. It also makes possible the to-and-fro motion of the lens necessary in the focusing.

To avoid spherical aberration, i.e., the distortion of the image because all the rays do not come to the same focus, and to limit the quantity of light, a diaphragm is placed in front of the lens. This diaphragm restricts the rays to those which pass through the central portion of the lens. The smaller the diameter of the hole in the dia­phragm, the better is the definition of the image. On the other hand, the smaller this opening, the less the intensity of the light forming the image on the plate. Where it is desired to have the image as bright as possible, the choice of the size of the opening in the dia­phragm becomes a compromise between diminished brightness on the one hand and definition on the other hand.

Condenser

Y

X

Figure 41 - Projection lantern. It produces real, inverted,

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