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.
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.
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
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