Digital Camera Shutter

The Focal Plane Shutter

This type of shutter blocks light on almost the same plane as the film, interposing a dark backdrop made up of two small metal or cloth curtains. It completely covers the exposure window , which limits the format. This is used in some direct viewfinder cameras, all 35 mm SLR and some medium format SLR. Its size and vibrations increase with the format, which is why it is rare in medium format and incompatible with large format.

Its work the following way: on both sides of the exposure window there are two foldable curtains. Each one can move from left to right (or from top to bottom, depending on the model) of the window until completely closing it. If the moving ends of the curtains are together, then the shutter is closed.

(1) When the camera is ready to shoot (loaded shutter), the curtains are together on one side of the window: one of them covers the window and the other is rolled up. (2) When the trigger is pressed the curtain that is covering the window moves back and rolls up on the other side (3) leaving the film uncovered during the selected amount of time; next, the second curtain extends (4) and meets its partner at the other side of the window (5). When reloading the shutter (6), we again take the two curtains, together this time, to the initial position.

The exposure time is achieved by regulating the time that passes between the arrival of the first and second curtain. For any exposure speed, the speed the curtains move stays constant. With long speed, the second curtain "waits" to go out at the required time, while the first curtain already arrived: in this case the exposure window stays open for an instant. On the other hand, with short speeds, the second curtain has to leave when the first one is still moving. The window never opens all the way an the exposure is made through a gap that moves from one side to the other. This system can reach very high speeds (higher than central shutter systems). The main advantage of this kind of shutter is that is installed inside the camera, and is compatible with all interchangeable lenses, which reduces their prices, weight and fragility.

The largest inconvenience is it's size, vibrations and noise, which increases with the format. In limited cases this vibration can cause moved pictures. Also, the high speeds aren't compatible with the flash because the shutter isn't completely open at any moment (9.4).

Mechanical and Electronic Shutters

It's important to remember that the shutter is a mechanism that doses time and is made up of it's moving parts, it's regulators and it's speed selector. The connection between the selected speed and the real exposure time is controlled by a n either mechanic or electronic regulator. The mechanical shutter is regulated by clockwork. When loading it we shift the moving parts and cock the regulating mechanism, made up of gears combined with the variable pressure exerted by a escape spring .

In an electronic shutter the moving parts are reduced to a minimum and the time regulation is electronic. A small quartz oscillator substitutes the gears and spring system, and to pass from one speed to another we don't have to move any gears. This makes an electronic shutter ideal to work at the orders of the photometer, who in automatic exposure setting, has easy access to the internal speed selector. The range of speeds can be amplified greatly without much mechanical complexity. The only inconvenience of this kind of shutter is that it needs electric power, therefore it doesn't work without battery power.