What is Shutter Speed?

Shutter speed is the amount of time the camera's shutter stays open, allowing light in, before taking a shot. It is usually measured in fractions of a second so is seen as a fraction like 1/125, or 1/2000. The larger the denominator, the faster the shutter speed (because the fraction makes up a smaller part of a second).

The standards figures 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, and a few settings for super-long exposure.   Each increment (apart from the super-long exposure) roughly doubles or halves the amount of light entering a shot. The fastest shutter speed available is 1/16,000 which is found on high quality DSLRs.

A fast shutter speed (eg 1/16000-1/1000) means that there is little time to let light in, so the picture will appear darker. This setting is suitable if you're in high light conditions or have other settings on the camera set to allow light in.

A slow shutter speed (eg 1/125 - 1/2) gives more time for light to get into the camera, so the shot will appear lighter and brighter.

Because of it's relationship with light, shutter speed is also known as "exposure time". A fast shutter speed is a short exposure time and a slow shutter speed is a long exposure time.

Effect of shutter speed on exposure vs the effect of aperture size on exposure

If you compare how much brighter a shot is made by a low shutter speed or by a large aperture, you find that doubling the aperture has a more dramatic effect on light than shutter speed:

• halving the shutter speed doubles the exposure (ie it increases the exposure by a unit of 1 EV, where EV stands for exposure value ), 
• halving the f-number (ie. doubling the aperture) increases the exposure by a factor of 4 (or by 2 EV units).  

Shutter speed and aperture have such a closely linked relationship that you could effectively get the same exposure of the shot at wildly different settings, by  considering this relationship. For example, all the following settings will have roughly the same exposure:

• shutter speed of 1/250 s and f/8 
• shutter speed 1/500 s and f/5.6, 
• shutter speed 1/125 s and f/11.  

Advantages of fast shutter speeds

• It allows more light into a shot which can be useful if you need to boost exposure without changing aperture or ISO.
• It allows you to capture things in motion without a motion blur. The fast shutter speed snaps a super-brief moment in time, so it can capture a movement that occurs in a fraction of a second. The less time there is to allow light to enter the shot from the next part of the movement, the less blur there will be. This can be useful in capturing wildlife mid-flight or mid-run, catching subjects in races and sporting events, and generally capturing anything in motion. For example, if you want to capture someone mid-jump with both feet in the air, fast shutter speed is ideal.

Advantages of slow shutter speeds

• Creating artistic motion blurs: This can be fun if you're using flashlights or photographing moving cars at night. It can create almost abstract shots.  It can also create magical looking effects like when you shoot running water with slow shutter speeds, the resulting motion blur gives the illusion of flowing fog sometimes. The amount of blur depends on the speed at which the object is moving, as well as on the distance the object is from you and the amount of zoom you're using (or focal length). Zoom accentuates motion blur.

Disadvantages of slow shutter speeds

• It's very easy to get a blurry shot even if you don't intend to. Any movement of the camera can result in a blur. It's really advisable to use a stabilizer like a tripod to steady the camera and prevent camera shake in anything slower than 1/60. 

What happens if you change the zoom whilst a picture is being exposed with a slow shutter speed? 

This is a photographic trick called "zoom burst" where you set the camera to a long exposure/ slow shutter speed, and whilst the shutter is open, you zoom in on the object you want to capture. The result is a sharp object in the centre (as long as it is still and not moving during the long exposure), and a radial blur in the surroundings from the motion of the zoom.