The next facet of exposure to talk you through is shutter speed. In different cameras this can be a slightly different thing, but in practice the shutter is a blind that opens and closes for a period of time to let light through.
The length of time a shutter is open for is the shutter speed. It’s measured in fractions of a second, and just like aperture, the values go up and down in stops:
The Shutter Speed Sequence
< Slower/more light gets through———-Faster/less light gets through >
Moving from 1 second to 1/2 second shutter speed halves the amount of light that can pass through, and, you’ve guessed it, is a change of one stop. Ten points for guessing what I’m about to say next – moving from 1/250s to 1/125s allows double the light through, and is one stop as well. Simple really. By now you also know that the shutter speed you choose is going to affect, and be affected by your aperture and sensitivity/ISO combination, but we’ll dig into this in more depth later on.
As with aperture, DO NOT PANIC about the fact that the numbers don’t quite follow a mathematical sequence. This doesn’t matter AT ALL in practice!
In some modern cameras, as well as all camera phones, the shutter functions differently to how I’ve just described. Rather than a set of blinds that open and shut, a processor essentially turns the sensitivity of the sensor up or down for a specific period, and this becomes the shutter speed. Whilst there are some differences between these 2 methods, they don’t need to concern you at this stage, and they’ll only really come to light when you start using flash or shooting video. I’m only really mentioning this for the sake of thoroughness, in case the geeks at the back are getting anxious that I’ve not dealt with 21st century kit, and am still stuck somewhere in the Victorian era.
Shutter speed affects movement, as the video above demonstrated with that tennis ball. In summary, the main creative effect shutter speed has is on the amount of movement in an image. A slow shutter speed means that any movement in an image, such as a car driving past, or a child running, will be blurred, and a fast/short shutter speed will freeze movement.
That’s it in a nutshell really. If you need to catch some motion and want it to look sharp, choose a fast shutter speed, and if you want to show movement, use a slower speed.
As you can imagine, there is a bit more to it than this. For one thing, you’ll be surprised at how fast you actually have to have your shutter speed to freeze even rapid human motion such as running, let alone something like a motor bike. To really freeze someone who’s running or jumping fast, you’ll need a shutter speed of less than 1/500s, and ideally even shorter. At the other end of the scale, if you’re getting creative and trying to use blur in your image, you’ll need to experiment with just how slow to go. You may find for example that even a speed which to us may seem fast – namely 1 second – is still too long for someone walking – they’ll just come out as a ghost, rather than the dramatic trail of movement you imagined. Play around with this yourself to see what works.
There’s one more thing to bear in mind with shutter speed and movement, and that’s your own movement. If you’re handholding your camera, rather than putting it on a tripod or resting it on a nearby wall or convenient relative, then there’s a point below which your image will be blurred simply by the fact that you can’t hold the camera as steadily as you think you can. Any camera movement whilst the image is being taken will cause the whole image to look out of focus. This is called camera shake, and that’s a pretty descriptive term really – it does what it says on the tin
I’ll be covering this in more detail later on, when I talk about focusing, but here’s a very quick example of what it does. Have a look at these two images, which are both blurred. In this image, there is actually some of the shot that’s in focus – look at the bellows towards the back of the camera. That’s because the blur in this shot is caused by simply focusing on the wrong part of the frame. In this image however, nothing is in sharp focus, as this blur is caused by camera shake. The best way I find to imagine it, is if you picture the light streaming in through your lens, and then hitting your sensor to “draw” the image, then if that sensor is moving around, the image will be drawn in a very shaky way.
In order to keep your image nice and sharp if you’re hand holding your camera, you need a shutter speed at least as fast as the length of the lens you’re using.
What on earth is he talking about now? Put simply, if you’ve got a 100mm lens on the front of your lovely new camera, you should be at a shutter speed of 1/125s or faster in order to avoid shaking the camera. For a 50mm lens, 1/60s or faster, and so on. The only way around this is to mount the camera on a tripod, which has both advantages and disadvantages – for the sake of brevity I won’t go into them here.
One or two keen-eyed viewers will at this point say “hang on, none of that matters anymore if you’ve got a lens or camera with some sort of image stabilisation built in.” This, unfortunately, is only slightly true. Image stabilisation is a very clever device whereby gyroscopes and motors are built into the body of a camera or lens, coupled to a computer, and then engaged in such a way to counteract the shakes your body makes at slow shutter speeds.
First off, the most you’ll gain is 2-3 stops, no matter what any manufacturer says. Secondly, and much more importantly, whilst image stabilisation can reduce any blur in the image from you shaking the camera, if you try and shoot a runner going past you at, say 1/15s shutter speed, they’ll still be blurred. Obviously, all the clever stuff in the lens has no effect on how fast the runner is moving, and you’ll still need a fast shutter speed. Image stabilisation, vibration reduction and so on, can only have an effect on the camera – not the rest of the world!
Right, that’ll do for now on shutter speed. Don’t forget it’s tied into aperture and ISO, so unfortunately you’re not completely free to choose the speed you’d like all the time. ISO is coming up next……