Aperture

In the first in a series of starters’ guides for new DSLR users, we explain the principles of aperture and f-stops

“Aperture is also known as ‘f-stop’ and you can think of it as being similar to the pupils in your eyes.”

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.

A particular aperture setting is referred to as an “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. These values are the result of dividing the aperture’s diameter by the focal length of the lens. Therefore, a 50mm diameter aperture on a lens with a focal length of 200mm has an f-stop of 1/4, which is generally written as f4, F4 or 1:4.

The standard seven aperture values range from f/2.8 to f/22 but below you will see two even wider f-stops, which are also commonly used for very shallow depth-of-field. Some lenses have smaller apertures than f/22 (e.g. f/32 or even f/45).

(wide aperture = shallow depth of field … narrow aperture = large depth of field)
(wide open) f/1.4 f/2.0 f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 (small opening)

The seemingly arbitrary numbers are quite confusing as the larger the number, the lower the amount of light entering the camera. However, if you remember that they are fractions then it starts to make sense. A half is bigger than a quarter, which is bigger than an eighth and so on.

Getting your aperture settings right

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.

Have a look at the two images below taken on a Canon EF 50mm f/1.4 USM lens. Although this lens can open wide to f/1.4, the photographer who took these photos didn’t want such a narrow depth of field. He couldn’t make his mind up between f/5.6 and f1.8. Which one do you prefer?

This image was taken at an aperture setting of f/5.6. Note how the subject appears completely in focus and the curtain in the background is only slightly blurred. This image has a medium depth-of-field.	This image was taken at f/1.8. The subject is in sharp focus but the foreground screwdriver is a bit more blurred and the background curtain is 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.