The shutter in your camera is a mechanical mechanism that opens and closes to expose the sensor to light. We measure the period of time it's open and the sensor is exposing a photograph, and call it a shutter speed. As we know, the aperture and the availible light determine how much light the shutter has to work with. When there’s not a lot of light to work with we tend to need longer shutter speeds, while in brighter conditions we can use a shorter time duration to expose our photographs.
There’s more to it than that though. The shutter speed if slow, will show motion, such as a milky waterfall scene, and if set fast will stop decisive action like a football player catching a touchdown pass.
Let’s examine the above photograph as an example. First, you will notice that I have a lot of bird pictures and need to get out more. Second, it should make sense that I needed a fast enough shutter speed to capture the hawk’s wings in motion. This means I used a small sliver of time.
We write shutter speeds as fractions of a second, such as 1/125 or 1/30. When you see a number and a quote mark it indicates full seconds, such as 30” to refer to 30 full seconds. Many cameras only give the denominator of the fraction on their LED screens, so 30 would be 1/30 of a second and 30” would be 30 seconds. You do not want to confuse these while shooting.
In the above photo I was trying to get crisp movement of the turtle, so even though there was very little light, I shot it at 1/200. Naturally, that was more difficult to do, since there was very little light.
The trick to using faster shutter speeds in lower light conditions is to have a fast maximum aperture lens and a cursory understanding of ISO. But that’s a subject for another blog post.