The shutter is a mechanical device to control the timing when the image sensor is exposed to light, and for how long. Controlling the duration, the shutter is one key element in a camera responsible for the correct exposure of the photograph. Some digital cameras may have purely electronic shutters, but this chapter focuses on the mechanical shutter mechanism found in DSLR cameras.
The mechanical shutter is located in front of the image sensor so that it can physically block incident light from reaching the sensor. It contains two sets of shutter curtains that can either cover the image sensor or retract from the frame in order to expose the image sensor to light. The first curtain is attached to the lower part of the frame and the second curtain is attached to the upper part of the frame. The initial position (when the camera is in its idle stage) is that the first (lower) curtain is blocking the image sensor and the second (upper) curtain is retracted. The curtains are springloaded when in their initial position. Once released, they can snap into their final positions within milliseconds. The shutter unit can release both curtains at different times so that the image sensor can be exposed to light for different lengths.
That type of mechanical shutter which is located directly in front of the image sensor is called a focal plane shutter. It should be noted that there are different other concepts for mechanical shutters such as leaf shutters which are located inside the photographic lens. However, these are not described here.
The full shutter firing sequence includes the following steps:
When being released from their ready-position, both shutter curtains move at the same speed. This ensures that the individual pixels on the image sensor get exposed to light for an identical amount of time. The timing between the release of the first and second curtain is what determines the overall time the image sensor is exposed to light. The image summarizes the shutter firing sequence.
In addition, covering the image sensor from incoming light allows the sensor to be read out in the dark. This eliminates over-exposure to light due to photons still hitting the photodiodes while being read out.