Modern 35mm cameras make use of a ‘shutter’ also known as ‘focal plane shutter’. The shutter is basically a lightproof curtain that blocks the light coming through the lens and hitting the sensor. The curtain is a two piece mechanism that overlaps a little. While taking photographs these curtains open momentarily allowing the light to hit the sensor. These curtains are called shutter and you can set the time for which you want the shutter to remain opened. This time duration is called shutter speed.
The agreed standards for shutter speeds are – 1/1000 s, 1/500 s, 1/250 s, 1/125 s, 1/60 s, 1/30 s, 1/15 s, 1/8 s, 1/4 s, 1/2 s, 1 s. Consider 1/1000; this means that the shutter speed is open for one thousandth of a second. Just like f-stops, there is a relationship between successive values.
Each shutter speed remains open half as long as the one above it. Slow shutter speeds are extremely helpful in underwater photography, due to low light conditions. Slowing the shutter speed is to extend the time until the shutter closes which in turn increases the amount of light gathered. Hence, faster the shutter speed, lesser is the light that comes in.
Besides light, shutter speed affects the motion of the image as well. A slow shutter speed will blur the motion whereas a faster one will freeze it. For instance- suppose you are shooting a fish with a shutter speed of 1s. This means that the shutter is open for a full second, meanwhile the fish will swim right through the frame. In this photograph the entire path of the fish will be recorded and as the fish was moving, it will be a blur. Now consider the same setting with shutter speed of 1/250 s. This will capture the fish at a particular point, freezing the motion.
One must also remember to hold the camera still while the shutter speed is low. (Note- one can struggle to hold the camera steady in water for long. Hence if the shutter speed is more than 1 sec then it will cause the subject to blur). If the camera is shaking while low shutter speed is used then the photo will come out blurry. Mechanical shutters have maximum shutter speeds that work on internal or external strobe known as strobe sync speed. If you use the strobes and set faster shutter speed then a partially black photo is seen. This is due to the slow curtain while the strobe was firing.