DSLR starters' guides: Aperture

Fri 29 February

In the first in a series of starters' guides for new DSLR users

What is aperture?

Aperture is the size of the opening in your DSLR's lens which helps determine the amount of light that reaches the camera's sensor.

Aperture is also known as “f-stop” and you can think of it as being similar to the pupils in your eyes. In bright sunlight, the irises of your pupils contract to restrict the amount of light hitting your optic nerve, while in the dark they expand to let as much light through as possible. Aperture - or f-stops - on a DSLR work in exactly the same way.

Aperture values are not absolute measurements but are relative values - the result of dividing the aperture's diameter by the focal length of the lens. So a 50mm diameter aperture on a lens with a focal length of 200mm would have an f-stop of 1/4 - generally written as f4, F4 or 1:4.

The standard seven aperture values are:

f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22

The arbitrary aperture numbers can be quite confusing as the larger the number, the lower the amount of light entering the camera. But if you remember that they are fractions then it starts to make sense - a half is bigger than a quarter which in turn is bigger than an eight and so on.

Top Tip:

Small f-stop = larger opening = more light entering camera

Large f-stop = smaller opening = less light entering camera

When playing with the aperture settings on your DSLR, you’ll probably see lots of other numbers below, above and between but the 7 we've mentioned are really the ones you want to concentrate on. Why? Because with each stop you move, you are changing the amount of light entering the camera by the same amount, doubling or halving depending on the direction. For example:

By moving from f2.8 to f4, you are decreasing the light entering the camera by one stop or halving the exposure.

By moving from f8 to f5.6, you are increasing the light entering the camera by one stop or doubling the exposure.

Why is aperture important?

Have you seen a portrait where only the eyes are in focus and the rest of the image is blurred? This is done simply by aperture. By choosing a small f-stop you can isolate your subject from the background. This is known as narrowing the depth of field.

Expanding on the tip above:

Small f-stop = larger opening = more light entering camera = isolates subject from background
Large f-stop = smaller opening = less light entering camera = blends subject into background

Have a look at these two images:

This image was taken at f/5.6. Note how the subject , the background and the foreground all appear in focus. This image has a deep depth-of-field This image was taken with an aperture setting of f/1.8. The subject is in sharp forcus but the foreground and background are blurred. This picture has a shallow depth-of-field

For portraits, there’s no denying that using small f-stops gives a certain punch to a picture. There are practical reasons for using large f-stops though. Landscapes, architecture and group pictures benefit from having a large part of the image in focus.