Understanding Aperture

In photography, the aperture is the circular diaphragm inside the camera (or if you have an SLR, inside the lens), that opens when you press the shutter release. When you set the aperture before you take a photo, you are telling the camera to what diameter the aperture should open when you press the shutter release.

Aperture settings are expressed as a lower-case ‘f’ over a number; for example f/8 or f/22. Sometimes, the slash is omitted, for example ‘f4’. Sometimes the f is omitted as well – this is common on camera LCD screens. For example, f/8 might be expressed as simply ‘8.0’. Confusingly, the lower the f-number, the larger the aperture. In other words, setting the aperture on your camera to f/1.8 means that the aperture will open very wide, whereas f/22 means the aperture will hardly open at all. Setting a camera to a small f-number is sometimes known as ‘stopping down’.

Aperture is one of two variables (the other is shutter speed) that determine how much light hits the film or the sensor inside your camera when you press the shutter release. Darker conditions will usually require a slower shutter speed, a larger aperture, or both, in order to get a good exposure.

If you put your camera into automatic mode (this might be represented by a green square or the word ‘AUTO’ on the dial on top of the camera), then your camera will determine how large the aperture should open by itself. If you put your camera into manual (an M) or aperture-priority mode (an A or Av), then you will select it yourself.

Like shutter speed, setting aperture affects more than just how light hits the film or sensor. Crucially, aperture setting determines what is called ‘depth of field’. At a small aperture, almost everything in a scene will probably be ‘sharp’ (i.e. in focus) – from whatever is nearest to the lens to whatever is farthest. At a large aperture, only what is focused on will be sharp, and everything that is in front of or behind the subject will be blurry.

One final thing: if you own an SLR, you’ll see that f-numbers are also quoted in lens descriptions. For example, in the description: ‘Canon EF 85mm f/1.8 USM Medium Telephoto Lens’. In this context, the f-number refers to the maximum aperture to which the lens can open. In the example, the lens can open to a very large aperture (f/1.8) indeed. Very few lenses open this wide, and fewer still open wider. Most consumer lenses are around f/4 or f/5.6. Some consumer zoom lenses have a variable aperture range, which means that they don’t open as wide at the telephoto end of the zoom range, for example: Canon EF-S 55-250mm f/4.0-5.6 IS Telephoto Zoom Lens.

Large aperture lenses are better because they are capable of taking photos in less light, and because they allow for the creative use of large apertures in giving a shallow depth-of-field, but the trade-offs are greater size and weight and greatly increased cost (both because large apertures require more glass in the lens).