All cameras have a shutter mechanism to expose the sensor for exactly the right amount of time. The shutter opens to begin the exposure and then closes again to end it.
Most cameras have mechanical shutters, though you can also get electronic shutters, which we’ll come to a little later. Mechanical shutters come in two types: focal plane shutters and in-lens (leaf) shutters.
Focal plane shutters
Focal plane shutters are found on interchangeable lens cameras like DSLRs and mirrorless cameras. These are positioned immediately in front of the camera sensor and although the mechanisms themselves may involve many moving parts, the basic principle is quite simple. They consist of a ‘first curtain’ which moves out of the way to expose the sensor and a ’second curtain’ which moves across from the opposite side to cover it again. After the exposure, the shutter is reset and the first curtain again covers the sensor ready for the next shot.
The length of time the sensor is exposed by the shutter is the shutter speed, which is usually measured in thousandths of a second.
You might hear these terms in flash photography, where they describe the point in the exposure where the flash fires. Usually, ‘first curtain’ flash is used – the flash is fired at the start of the exposure. Sometimes, ‘second curtain’ flash is used at the end of the exposure to achieve a particular visual effect.
At slower shutter speeds, the whole sensor is exposed at once as the first curtain clears it completely and there is a momentary delay (depending on the shutter speed) before the second curtain covers it again.
The only way to achieve higher shutter speeds is to release the second curtain while the first curtain is still moving. Now, the sensor is exposed through the moving slit between the two moving shutter curtains. In this way it’s possible to achieve shutter speeds as high as 1/8000sec.
For flash photography, however, the whole sensor must be exposed when the flash fires. There is a maximum shutter speed at which this still happens, typically around 1/200-1/250sec. This is the camera’s maximum flash sync speed. With some flash systems it’s possible to get around this limitation with high speed sync systems that either ‘pulse’ the flash repeatedly or extend the flash duration to cover the time it takes for the slit between the shutter curtains to traverse the sensor.
In-lens (leaf) shutters
In-lens ‘leaf’ shutters use a diaphragm mechanism similar to the lens’s aperture mechanism, which opens and closes rapidly to start and end the exposure. (On some cheaper cameras the shutter and aperture mechanism are combined.)
Leaf shutters can expose the whole sensor right across their full shutter speed range, which makes them very good for flash photography. However, building a shutter into every lens is expensive and they can’t match the high shutter speeds available with focal plane shutters.
Many cameras now come with ‘electronic shutters’ . These effectively switch the sensor on and of electronically rather than shielding it with a shutter mechanism, and are offered as an alternative to regular mechanical shutters.
An electronic shutter can offer a higher maximum shutter speed, typically 1/16,000sec or even faster, though this is not all it seems. In fact, the sensor data is ‘scanned’ strip by strip rather than being read all at once, so although the effective shutter speed is very short, it still takes time to read the whole sensor area.
During this time, the position of fast-moving subjects may change, giving them a skewed appearance in the final image. In stills photography, this means that although electronic shutters allow wide lens apertures even in bright light, they are not well suited to capturing moving subjects or fast action.
In video, the shutter is electronic, and this ‘skewing’ effect with moving subjects (or with rapid camera panning movements) is called ‘rolling shutter’. The only way to prevent it is with an electronic shutter than can capture the data from the whole sensor area in an instant. This ideal electronic shutter is called a ‘global shutter’, and while prototypes exist and are in development, it requires a very fast sensor readout and is not yet found in mainstream cameras.