The shutter speed determines the length of time the shutter remains open to expose the picture.
Shutter speeds are incremental and are always displayed in fractions of a second.
The table above shows shutter speeds in full exposure stop increments.
When the shutter speed is increased by 1 full stop (1/125 – 1/250) the amount of light hitting the camera sensor is halved. Similarly if the shutter speed is decreased by one full stop (1/250 – 1/125) the amount of light hitting the sensor is doubled.
Shutter speeds are typically used to control movement in an image. For example, a slow shutter speed will show motion in a picture. The image below was taken at 1/25s. As you can see the waves are slightly blurred showing that they are moving.
The next example shows how a fast shutter speed freezes movement.
Shot at 1/1600s. As you can see the dog’s movement is frozen and you can see every droplet of water as it splashes up.