The shutter
Bob Newman gets to grips with shutter technology, from leaf and focal-plane to electronic shutters
‘With a few exceptions, shutters have conformed to one of two basic designs’
It is a requirement in photography to be able to control the exposure time. In the early days, when film materials were slow, the exposure time would be measured in seconds or even minutes. In those days, the ‘shutter’ was also the lens cap. To start the exposure the photographer simply took it off, then counted ‘one elephant, two elephant…’ When the number of elephants reached the number of seconds for the exposure, the lens cap was put back on.
As emulsions and lenses became faster (so allowing a faster shutter speed), it required a nimble hand to reliably achieve the fractions of a second required and cameras began to be equipped with a mechanical device to control the exposure time. With a few exceptions, shutters have conformed to one of two basic designs. First is the leaf shutter, which works like an aperture iris but can close completely (in fact, in some lower- end cameras the shutter and aperture mechanisms were combined). To operate, the shutter opens from completely closed, then after the required time has elapsed, closes again. The second model is the focalplane shutter, which sits just in front of the image plane and has two blinds that move at a constant speed. The space between the two blinds controls the exposure.
Both forms of shutter have pros and cons. The focal-plane shutter makes the design of an interchangeable lens much simpler. Also, it can allow short exposure times by using a narrow space between the opening and closing blinds – so long as the speed of the blinds is well controlled – as short as 1/8000sec or less.
The disadvantage of this is that at high speeds the shutter is never completely open, so the maximum shutter speed with flash is the fastest that exposes the whole frame, at which point the flash must fire. Second, since the exposed stripe moves across the frame, a subject that moves during the exposure will be distorted.
The leaf shutter has neither of these problems, but has some of its own. It needs to be placed near the optical centre of the lens, which means that in an interchangeable-lens system, each lens needs its own shutter. Since the shutter blades have to move, stop, reverse direction, then move and stop again, the motion is much more complex than for a focal-plane shutter, so the maximum shutter speed attainable tends to be slower.
Since the advent of digital photography a third option has become available – the electronic shutter. This is realised by manipulating the way a sensor works. Before each exposure, the sensor is cleared of the charge from the previous exposure. As soon as it is cleared, it starts accumulating charge so the clearing operation marks the start of an exposure. After the exposure, the charge is read off the sensor, stopping the exposure. By controlling the time between the clear and read operations, the exposure time can be controlled.
Most sensors clear and read the sensor line by line, so the normal electronic shutter has properties similar to a focal-plane shutter, with the electronic ‘slit’ moving down the frame. By introducing an extra transistor in each pixel, this can be converted to a ‘global’ shutter, where the whole sensor is cleared and read at the same instant. This provides the shutter with some of the properties of a leaf shutter, at the cost of extra complexity, and therefore a little less efficiency in the sensor. The disadvantage of electronic shutters is that their action seems to introduce extra electronic noise into the image.