Aperture: The Pupil Of Your Camera’s Eye

The aperture is like the pupil in a human eye – it’s the hole in the lens which permits light to enter through to reach the camera’s sensor. The size of the aperture can be changed – just as a pupil shrinks in harsh sunlight and expands in the dark. A wider aperture lets more light in (causing a brighter exposure), while a narrower one restricts the amount of light that enters (for a darker exposure).

Increasing the size of the aperture to let more light into the camera has a side effect – less of that light will be focused. This means that areas in front of and behind the point in the scene which the camera is focused on become more out of focus the further they are away from that point. This effect is commonly used in portrait photography to draw the viewer’s attention to a key part of the image, usually the subject’s eye(s), while the foreground and background fall out of focus. Wider apertures also enable you to take portraits in lower light levels without having to use flash.

Setting the aperture

Changing the size of the aperture is done by setting different ‘f’ numbers. Switch your camera into Aperture Priority mode (usually marked as ‘A’ or ‘Av’) and turn the main control dial to view the aperture settings available to you with the lens you are using. Confusingly, the wider the aperture, the lower the ‘f’ number. So a setting such as f/1.4 or f/2.8 will let much more light in than a setting at the other end of the spectrum, such as f/18 or f/22.

The amount of the image which is in focus is referred to ‘the depth of field’. An image with a low ‘f’ number will have a wide aperture but a very shallow depth of field. Conversely, an image with a high ‘f’ number will have a narrow aperture but a wide depth of field. At the extreme end, such as f/1.4, the depth of field could be as little as 1cm, so accurate focusing becomes essential. Any slight movement of your camera or your subject between setting the focus and taking the shot could result in your subject’s eyes becoming accidentally blurred.


Bokeh is a Japenese word that translates most closely as ‘blur’ and is used when discussing the aesthetic quality of out-of-focus areas of photographs. Although a subjective judgement, ‘bad’ bokeh tends to refer to out-of-focus objects in the background which have harsh outlines, or when blurred points of light are captured as edged polygons rather than smooth circles. ‘Good’ bokeh is when the background is blurred so it is smooth, creamy and complementary to the overall image, rather than an unwanted distraction. If, for example, you are photographing someone at dusk, and there are coloured lights in the background, using a good quality prime lens at its widest aperture will ensure the lights become beautiful, soft circles of colour glowing behind your subject.

Lens limitations

Cheaper lenses aimed at amateurs, or those provided in a ‘kit’ with enthusiast cameras, simply don’t offer the wider apertures that are available on prime and professional lenses. Therefore you may find that the widest aperture you can get on one of these lenses is f/3.5 or f/5.6, which means it’s harder to throw the foreground and background out of focus. To achieve a similar effect, you can step back from your subject and zoom in. The further away your subject is from the background, the more blurred it will become.