SHUTTER SPEED IS one of the three basic controls used to ensure that the correct amount of light is recorded by the camera and, as such, controls how light or dark the final image will be.

The shutter is a set of metallic blades, which is normally situated between the camera's sensor and the lens.

When a picture is taken, the shutter opens and allows the light falling through the lens to reach the sensor. The period of time for which it remains open is referred to as the shutter speed.

This speed is variable and by lengthening or shortening it, the camera or user can adapt for changing light intensity.

The longer the shutter remains open, the more light is transmitted through the lens. So if it is very bright and sunny, you would normally use a very short shutter speed, and in low light, a much longer shutter speed, other things being equal.

Almost all modern cameras can automatically adjust this speed, but, as the pictures on the right show, it is a powerful tool and is best manipulated by the photographer.

By determining how long the shutter remains open, we are in effect also deciding how much the reality in front of the lens will change while the sensor (or film) remains active. This allows us to introduce an element of blur into the photograph, and a touch of blur, well used, can add visual depth and a sense of movement that is not achievable through any other means.

Of course, too much blur can completely ruin a photograph. The trick is to manipulate the speed so that it is appropriate to your desired end results.

How to adjust the shutter speed

Most digital SLRs and some compacts have at least the following three shooting modes: Manual (denoted by a large M); Shutter Priority (a large S or Tv) and Aperture Priority (you've guessed it, a large A or Av). The first two of these will allow you to manually adjust the shutter speed. In Manual mode you will have to set both the shutter speed and aperture for an accurate exposure, while in Shutter Priority mode you only have to set the speed, the camera will set the aperture for you.

So let's assume you are in Shutter Priority mode. There should now be a button or dial on the camera that allows you to set the shutter speed. The speeds you can choose from usually include the following:250; 125; 100; 80; 60; 50 and so on, down to one, and then 2"; 4"; 6"; up to about 30"

The first of these numbers, those without the quote marks indicates that the shutter speed will be equal to one over the number indicated. So, if the camera's display shows 250, the shutter will remain open for 1/250th of a second; if it displays 4, for one quarter of a second.

Roll the number down to one. The shutter speed is now one second. If you press the button to take a picture (called the shutter release button), you will here a pronounced pause between the metal blades opening and closing. Now set it back up to about 250 and do it again. You will hear the difference.

The second set of numbers which are followed by a quote mark (10") denotes a number of seconds. So 15" means that the shutter will remain open for 15 seconds. There may also be a final setting which will read 'bulb' or perhaps 'b'. This merely means that the shutter will remain open for as long as you hold down the button.

If you are shooting in Shutter Priority mode and the shutter speed is too high or low for the ISO setting you are on, the camera will display an error message such as "lo" or "hi". Heed this warning and adjust accordingly, or your pictures may be too light or dark. (Of course, the camera's built in light meter is fallible and in a future issue we will look at how to override it completely).

How slow can you go?

The picture on the right is clearly blurred. This blur was not caused by an out of focus lens, but rather by what is known as camera shake. Of course, the longer the shutter remains open the more likely we are to end up with results like this.

But at what speed camera shake will rear it's ugly head depends on the length of the lens: the longer the lens, the larger the magnification of the image, but therefore also the magnification of every little movement in the camera.

The rule of thumb in the days of 35mm film cameras was to use a shutter speed faster than the length of the lens. In other words, when using a lens of 300mm you will need to open the shutter for less than 1/300th of a second. Because the sensor of DSLRs are generally slightly smaller than a 35mm negative, the magnification factor is higher, often as much as 50%, so the shutter speed will have to be increased by 50% too. So take the length of the lens, divide by two and add the length of the lens. 300mm ÷ 2 = 150. 150 + 300 = 450. So the new rule of thumb says for 300mm lens you will need a shutter speed of higher than 1/450th of a second.

It sounds a bit convoluted for a rule of thumb, so we'll put it like this: Your shutter speed should be a little faster than the length of the lens to avoid camera shake.

There will of course be times when you want the shutter to remain open for a longer period of time, and if this is the case, you could use some sort of support to steady the camera, such as a monopod or tripod. A monopod will quadruple the minimum amount of time you can leave the shutter open, while a sturdy tripod will make it possible to use any speed you like.

What is motion blur?

Look closely at the top left picture. It is also blurred, but this is not the same type of blur as in the picture above. In this case the camera remained static (it was mounted on a tripod). The shutter remained open for about two seconds. This allowed the subject in front of the lens to move, creating a blurred image.

Blur caused by movement in the subject as opposed to movement in the camera is referred to as motion blur.

The second picture on the left appears, at first glance, to be sharp, but on closer inspection, one can see that the guitarist's right hand is blurred.

In this case, the shutter remained open for 1/25th of a second. During this period the camera remained still and the musician hardly moved. His head and body was static, but the right hand traveled quite a distance, causing it to blur.

This localised blurring adds to the visual impact of the shot and communicates the energy on the stage.

Finding the exact point where only a part of the image is blurred and the remainder is sharp is, of course, a tremendous challenge, and it takes a long time before you can predict the correct setting, but this is such a powerful tool that it really is worth the effort.

Motion Blur and Flash

A slightly more advanced technique requires the use of a flashgun and camera that support rear curtain sync.

Rear curtain sync means that the flashgun fires a millisecond before the shutter closes rather than just after it opens.

This allows you to do what was done in the picture on the right. During the first part of the exposure, the flash does not fire and all that is recorded by the camera is the blur created by the person's movement. At the end of the exposure, just before the shutter closes, the flash is triggered and in this instant, the burst of bright light freezes the action.

The resulting image has a blurred trail and a sharp image at the end, it is very impressive when properly executed, but getting the balance between the strength of the ambient light and that from the flash just right is very tricky. This is certainly not a technique to use on once in a lifetime opportunities.

Other tricks with shutter speed

If you're wondering why we have not yet discussed panning, that's because we have a whole panning feature, which explains the technique and shows a few examples as well.

The image on the left was created using a technique called zoom burst. It's fairly simple. All you have to do is zoom the lens during the exposure. This will create a radial blur around the centre of the image. It is a limited technique, in that there aren't too many applications for it, but it is still handy to have in your repertoire.

Creating star trails is another fun, if limited trick. On a moonless and cloudless night, take the camera outside and set it up on a tripod, looking up at the stars but with something that won't move in the foreground, such as a church or a gravestone. Open the shutter for several minutes, perhaps half an hour or longer. The foreground image will record razor sharp, but as the earth turns during the exposure, the stars in the background would have moved, leaving curved and multicoloured trails behind.

Water and blur

True to form, we have saved the best for last.

By far the most impressive technique for those just discovering slow shutter speed photography, is the effect it has on water. Have you ever seen those images where it seems as if there is smoke on the water? That is what happens with exposures of about five seconds or longer of rapidly flowing water. Again, you will need a tripod for this photo, and you may have to dim the light which is transmitted through the lens by using a filter, but the results will astound you.