Some makes of this shutter have a speed indicator let into the india-rubber tube near the bulb, and when so arranged exposures up to several seconds can be given; the shutter will then close automatically. This action depends upon the escape of air through a small hole in the indicator.
The Iris shutter is a between-lens shutter, being fitted up near the diaphragm. It is made up of metal plates which work one upon the other. There are two kinds of this shutter, one requires to be set before the exposure, the other is "Ever-set." The first is set by pulling round a small knob, usually found at the top of the dial, A; this dial also sometimes carries the speed numbers of the shutter. In the illustration, these speeds will be seen on the plate near the bottom. The variations are made by moving the top indicator along. The bottom indicator governs the diaphragm apertures. The shutter is worked either by the trigger, B, or by an india-rubber tube and bulb fitted to the bottom of the cylinder, C. When the bulb is pressed, it lifts the inner cylinder, D, and the shutter plates are freed. The same result is obtained by pressing the trigger, B.
The speeds vary from 1/100th of a second to two or more seconds. When the indicator is over T, the shutter is opened by pressing the bulb, and remains so until further pressure is given. When over B on the plate, the shutter is opened by squeezing the bulb, and remains open as long as the pressure is maintained, but closes immediately it is relaxed.
The "SEE-SAW" Shutter, Fig. 28. The shutter here illustrated is the type usually met with on magazine hand cameras. The mechanism of the shutter in many instances is much more complicated than the illustration shows. The sketch was made from a shutter on a small hand camera and selected on account of its simplicity; it embraces all the essentials of a good shutter. It consists of three metal plates, A, B and C. The plate, A, is screwed to the camera, and has its edges turned to form grooves to receive B and C. The second plate, B, is pierced with a square hole; the third plate, C, is between A and B, and is indicated by the cross lines in front of and under the opening of B, and extends as far as the dotted lines only. It is called a "Cut-off," as it keeps the opening in B closed, while it passes in front of the lens. The little spring catch, D, holds B and C together. For instantaneous exposures the shutter is worked by pressing down the lever, E, the fork at the other end pulls B and C along, until they reach the projection F; this lifts the spring, D, and frees B, which is forced along by the spring, G; as its opening passes the lens the exposure is made. When the pressure is taken off the lever, E, the spring, H, draws C back to its proper position.
To set the shutter for time exposures, the arm, I, is turned by means of a little button on the outside of the camera (shown in Fig. 12, D); this pushes B and C along A. When the lever, E, is pressed, B is liberated by F and slides along until it reaches I; here it is checked, the lens uncovered, and remains so until the pressure is taken off E. C then closes the lens. Very long exposures are made by opening the lens and pushing E into a notch at the side near the bottom of the slot in which it works. The lens is closed when the necessary exposure has been given.
The shutter as illustrated has only one speed for instantaneous exposures, and that speed depends upon the strength of the spring G. Most shutters of this class, however, have a different kind of spring, which is regulated by the button, E, Fig. 12, to vary the speeds at which the shutter will pass the lens. These speeds may vary from 1/10th to 1/100th of a second.