An aperture is simply a hole in the lens – a variable diaphragm that can be made larger or smaller to control how much light reaches the sensor. You can control the aperture size using the dial on your SLR (or it can be set for you by the camera). The aperture size is measured on the f-stop scale. The relationship between the numbers on the scale can be hard to grasp. The best way to think of them is as fractions, with f/4 being twice as large as f/8, f/8 twice as large as f/16, and so on.

Your choice of aperture will vary depending on the lens you use, but it will generally range from a widest setting of around f/4 to a narrowest of around f/22. Aperture size is divided into so-called stops – f/4, f/5.6, f/8, f/11, f/16, f/22, and so on – with each stop effectively halving the amount of light reaching the sensor. As mentioned earlier, this can be compensated for by doubling the exposure time.

What does depth of field mean?

In addition to helping control exposure, your choice of aperture also affects what’s known as ‘depth of field’ (or DoF). The depth of field is a measure of how much of your photo is in focus, both in front of, and behind, the point you’ve actually focused on. Depth of field is more apparent when your image contains elements at varying distances from the camera, and is particularly noticeable in the background. Whether your background is sharp or out of focus depends on your aperture choice.

Setting a wide aperture reduces the depth of field, helping to separate the subject from the background

To blur or not to blur?

A wide aperture of f/2.8 will produce an image with a very shallow depth of field, meaning that everything behind or in front of your focal point will be blurred, which is great for portraits. On the other hand, a narrow aperture – f/22, for example – will maximise the depth of field, which is ideal for landscapes. Depth of field also varies depending on the focal length of your lens, and how close you are to your subject. The longer the lens and the closer you are to your subject, the shallower the depth of field at a given aperture.

Narrow aperture

Why use a narrow aperture?

• Increases the depth of field to ensure the whole scene is in focus, from front to back
• Captures the optimum degree of fine detail – ideal for close-ups and landscapes
• Enables you to use slower shutter speeds for creative motion-blur effects

Wide aperture

Why use a wide aperture?

• Reduces the depth of field to focus attention on your subject while keeping the background blurred
• Allows you to create arty abstract shots with only a few millimetres of the subject in focus
• Enables you to choose faster shutter speeds to freeze motion or stop camera shake ruining shots

Back to: Get better exposures

Forward to: Shutter speed explained

Like the aperture setting, the shutter speed you choose isn’t simply a way of controlling the overall exposure. It also has a visible effect on your shots, enabling you to control the appearance of a moving subject.

Fast shutter speeds freeze movement, ensuring pin-sharp pictures no matter how unsteady your grip on the camera, or how fast the subject is moving. Slow shutter speeds tend to blur movement, and so can be used for creative effects. Generally speaking, the slower the shutter speed, the greater the degree of motion blur in the image you take.

To freeze the motion of a fast-moving subject, choose a fast shutter speed

The shutter speed is the length of time that the shutter is left open. The scale used is easier to understand than the aperture system, because speeds are measured in fractions of a second. However, the numbers are often simplified – so 1/125 sec is shown as 125, 1/15 sec as 15 and so on. Speeds of a second or longer are shown as 1”, 2”, and so on.

Working in stops

As we’ve established, your aperture and shutter speed work together to capture an exposure. This means that there is no one single combination that will give you the correct exposure. You can pair a slow shutter speed with a narrow aperture, or a fast shutter speed with a wide aperture, and get a shot that is equally as bright. In the following sequence, 1/125 sec at f/5.6, 1/60 sec at f/8 and 1/30 sec at f/11 will all let in the same amount of light to produce an identical exposure.

Each f-stop number is ‘half’ the size of the one before it, and so lets in half as much light. The difference between two sequential f-stop numbers is often referred to as an f-stop, or simply a ‘stop’. If you reduce the aperture by one stop (letting less light in), to set the same exposure you need to compensate by slowing the shutter speed by one stop, to allow that light to hit the sensor for twice as long. Unless you’re in manual mode, your SLR will work this out for you.

Keep up to speed

If you’re shooting handheld (without a tripod) you’ll need a fast enough shutter speed to make sure camera shake doesn’t cause blurred shots. A good rule of thumb is to use a shutter speed that’s faster than the focal length on your lens; so with a focal length of 200mm, make sure you shoot at around 1/250 sec or faster.

Setting shutter priority mode (S/Tv) enables you to choose the shutter speed

The more you zoom in, the faster the shutter speed will need to be. Keep an eye on the shutter speed in the viewfinder and widen the aperture if necessary. In low light, you may need to increase the ISO, or use a tripod, which will allow you to shoot at very slow shutter speeds.

Back to: Understanding aperture

Forward to: Perfect your exposures