Shutter speed is shown in fraction form. The fraction is referring to the 1/15 of a second (slow) that it takes the shutter to open and close. Or the 1/6000 of a second (fast) it takes to open and close.
A slow shutter speed allows more light to come in because the shutter is open longer. But since the shutter is opened longer it also allows for some movement in your picture (motion blur) if it’s open longer than 1/80 of a second and there is a moving object/child in front of the camera.
Many photographers over estimate how steady they can hold a camera. Using a standard 50mm lens the minimum shutter speed you should use will be ‘one over 50′, which is 1/50. To turn it into a shutter speed you simply call it a fraction of a second, like this; 1/50th of a second. The nearest shutter speed you can set is actually 1/60th of a second but you get the idea. So if you’re shooting at 200mm the minimum shutter speed you should use will be ‘one over 200′, which is 1/200th of a second. So you would set your shutter speed to 1/250th of a second, that being the closest to 1/200. These figures are just a rough guide and may vary according to individual ability. Over time you will learn how slow a shutter speed you can hand hold but if in doubt always err on the fast side. It wouldn’t generally recommend you hand hold any camera/lens combination below 1/30th of a second.
The image below was shot with a shutter speed of 1/4 second (f20, ISO 100) using a neutral density filter. To get this effect on a very sunny June day was not easy. I had my son Theo stand still while his friends ran around him. The camera was on a tripod because of the slow shutter speed needed and I had screw-in neutral density filter +8 filter otherwise the image would have been completely over exposed as there would have been too much light hitting the sensor. There’s a great article on ND Filter’s here.
For most people the most important aspect of choosing a shutter speed is getting one fast enough to freeze movement. Unfortunately the world is full of things that all move at different speeds and no one bothered to make a list of what shutter speed would be needed to freeze each and every one of them. With this in mind I have put together a little table containing some common ‘things that move’ and a suitable shutter speed to stop them. You can use these values as a basis for working out shutter speeds for other situations you may encounter. This is a rough guide only so don’t sue me if one doesn’t work for you – try increasing another stop.