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CAMERA APPLIANCES
157
very short exposures are to be made. One kind of focal plane shutter has already been briefly described. Other shutters may be divided into two classes : (a) those which work in front of the lens or immediately behind it, and (b) those which are placed between the components of the lens system.
The Thornton-Pickard (see fig. 74) is perhaps the best known of the class (a). This shutter can be used for time or instantaneous exposures and can be fitted either in front of or behind the lens. In the latter case a loose panel is supplied to which the flange of the lens can be screwed.
One of the Thornton-Pickard shutters, known as the Foreground Shutter, is especially constructed so as to give more exposure to the foreground than the sky. This is a very useful one for obtaining cloud effects, and, in fact, for all landscape and seascape photography.
Other well-known shutters are to be found among the various kinds of sector shutters, such as the Goerz, the Unicum, and the Koilos.
Whatever kind is used, it should as far as possible be free from all vibration when in action. It should be possible to obtain a fair range of speeds, and the marked speeds ought to be accurate, or, at least, if they are too high, all the speeds should be affected in the same proportion.
The mechanism of the shutter, too, should be such that the liability to get out of order with proper use is reduced to a minimum, and the adjustments required should be of an easily understood nature. If required for such work as the photography of animals, it should make no noise
Fig:. 74.