The Camera Shutter
The camera shutter lets light in to the camera and on to the film or sensor. Film cameras use a mechanical shutter either in the lens or just in front of the film (called a focal plane shutter.) In modern camera designs the mechanical shutter may be driven by electromagnets rather than the springs used in older designs, and will normally be electronically controlled to give more precise timing.Digital Electronics
Digital cameras may use a mechanical shutter - most dSLRs do. Simpler cameras may use the camera electronics rather than an actual shutter. Basically the sensor has to be cleared, then allowed to build up a signal for the time required, and the signal then read again.Speed Sequence
When shutter speeds are set manually, they usually use the following sequence of speeds (in seconds): 1, ½, ¼, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000 etcEach speed in this series is (roughly or exactly) half that to the left, giving a halving in the amount of light reaching the film. Although the term 'stop' originally came from the camera apertures, photographers talk about this series representing 'one stop' differences.
Automatic speed setting may use the same discrete speeds, but is often is able to vary the speeds steplessly.
In lens shutters
Shutters in or just behind the lens are used in both expensive professional medium and large format equipment - where every lens has its own shutter - and also in consumer compact cameras. These shutters have a number of blades (the simplest only one) that open to let light in, then spring back into position to end the exposure. In some very simple cameras the same blades that create the aperture also make the exposure. These in lens shutters generally have a fastest speed of 1/250 or 1/500 of a second, but can be synchronized with flash at all speeds.Focal plane shutters
These have two blinds that run either horizontally or vertically across just in front of the film or sensor. Before the exposure, the first blind covers the film. To start the exposure, this runs across uncovering the film to light. The second blind then moves across to cover the film again.At slow shutter speeds, the first blind has finished moving before the second starts, so the film is completely uncovered for a short time, allowing flash to be synchronized. At faster shutter speeds the second blind starts when only a part of the film has been uncovered and the two blinds travel across exposing a slit of film between them. Flash at these speeds would only expose the film behind the gap between the blinds.
This exposure through a relatively narrow moving slit allows focal plane shutters to have very fast shutter speeds - up to 1/8000 s on some cameras, but flash sync is usually limited to a relatively slow speed, perhaps 1/125s. With some more modern camera and flash designs, higher flash synch speeds are possible.
