A shutter is an automatic mechanical device to open the light path through a camera for a predetermined and usually brief time, for the purpose of making a photographic exposure.
Most shutters also include a device to set the lens in the open position, either locking it open (âTâ for time) or allowing it to remain open for as long as pressure is applied to the release lever (âBâ for bulb, or nowadays, brief). In addition, all shutters provide for a very short âinstantaneousâ exposure, which may be of a fixed duration, say 1â25 or 1â40 second as in box cameras (Fig. 11.1), or varied at will from 2 seconds down to 1â1000 second or less in the more complex shutters (Fig. 11.2) or in focal-plane shutters.
An incredible amount of ingenuity has been applied to the mechanical design of shutters, from the simple flip-flop rotary shutters common in the earliest box cameras, through the more complex shutters equipped with flash synchronization that appeared after World War II, to the extremely accurate and reliable shutters made today. The timing of modern shutters is often electronically controlled. We shall not consider here the mechanical construction of shutters, but only those aspects that are of a purely optical character.
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