Contrary to what many people believe, shutter speed is not how quickly the shutter moves when it opens and closes; rather it determines how long the shutter is held open before it closes. And there are two things that happens when the shutter is held open: 1) more light gets in and 2) the risk of / opportunity for movement blur increases.
Wait - 'opportunity' for blur? Yes - when done well, blur can create wonderful photos. Think of images you've seen of recent Olympic track cycling events. The best ones will show the cyclists 'tack sharp' - as a clear, frozen image - with the background streaked. In this case, the more blurred the better as it gives a strong feeling of movement.
So how do you achieve this? By dialling down the shutter speed so that the sensors have more time to capture the image and 'panning' with the subject. This is also how photographers capture the silky effect on waterfalls (except for the panning part. That's a topic for another day). The problem with a lower shutter speed is that you run the risk of unwanted blur. There's not much point in a nice, streaky background if the cyclists are blurry due to camera shake.
The further you lower the shutter speed, the more you need to stabilise the camera and the most common way is to use a tripod. But you don't always have that option - some well-visited tourist attractions won't permit tripods, or you might not have one handy. When faced with this situation, find somewhere stable to place the camera, such as a tree stump or your car roof. Or, you could steady yourself: pressing the lens against the corner of a wall can stop movement just enough to deal with a slower than normal shutter speed.
Of course, there comes a point when it's simply too slow for hand-holding a camera. A safe rule of thumb is not to use a shutter speed that's lower than the length of your lens. Which means that if you're using, say, a 70-200 lens and you have it fully zoomed out to 200, you should set your shutter speed to a minimum of 1/200sec. Play with this to see what works for you - I've successfully taken photos at 1/15sec and then others that were blurry at 1/100sec. It depends on the situation - are you nervous about taking the photo or did you just run to catch the perfect spot? Then you're more likely to have shaky hands.
In addition to bracing yourself, there are other ways of keeping it steady. Shooting at 1/15sec is about where your heartbeat starts to cause camera shake. So the one thing you don't want to do (and which you should not do in any photography situation) is hold your breath. Because holding your breath causes your heart to pound and your hands move ever-so-slightly when your heart beats strongly.
So here's a tip you should use every time you take a photograph: breathe normally and calmly, then while breathing out, slowly and gently press the shutter. Continue pressing the shutter until after you hear both the open and close clicks, and continue to breathe out until after you release the shutter button. Practice doing this and you'll find you're doing it without thinking about it.
What are your thoughts? What methods do you use to prevent camera shake at slow shutter speeds?
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