Shutter

The shutter lets light pass into the camera for a measured amount of time. When exposure time is shorter than 1/125 s - 1/250 s the shutter is not able to expose the whole sensor to light at the same time. During short exposures on a modern camera the sensor is typically exposed in a strip moving from the bottom up. This means that fast moving objects and flashing lights may be distorted and be hard to interpret without a full knowledge of the way the specific shutter works. Some digital cameras have electronic shutters to overcome this limitation.

Maybe the largest problem arises when using an electronic flash as a fill in on a sunny day. The flash will refuse to fire because it is not able to light the whole scene at short exposure times. Some flash units have "high-speed sync" which means that they will emit a series of flashes at very high frequency to be able to light the whole scene as the shutter exposes the sensor bit by bit.