Curtain calls

The fine details of how shutter speed is controlled

Your Nikon’s shutter controls how long the sensor is exposed to light for. It uses a pair of ‘curtains’, or ‘blinds’, to do this (see page 82). The first opens to start the exposure, and the second closes to end it. At slower speeds, both curtains are open for some of the exposure. At faster speeds (anything above the ‘flash sync speed’ of 1/200 sec or 1/250 sec) the second curtain begins to close before the first has completed its travel. This means that the sensor is effectively exposed to light through a moving slit.

At the start of each exposure, the first curtain is fully closed and the second fully open. They are reset to this position by the camera at the end of every exposure, ready for the next shot. Using two blinds ensures that the whole image area gets the same amount of light. The top of the scene is exposed to the sensor first, but this part is also blocked off first by the second curtain. At the end of the exposure, the first curtain is fully open and the second fully closed.

Normal flash is only possible when the entire area of the sensor is exposed at the same time. This never happens with faster shutter speeds (shorter than 1/200 sec or 1/250 sec), making these the limit for normal flash operation.