Aperture explained: what f-stops really do
Aperture is the size of the opening inside your lens that lets light reach the sensor. It is written as an f-number, such as f/1.8 or f/8, and it does two jobs at once: it controls how much light gets in, and it controls how much of the picture is in focus.
What the f-number means
The f-number is the focal length of the lens divided by the width of the opening. That is why it looks backwards: a small number is a big opening. f/1.8 lets in much more light than f/8.
The full stops on the scale are f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22. Each step to the next number halves the light, and each step back doubles it. Cameras also offer thirds of a stop in between.
Aperture and background blur
A wide aperture (a small f-number) gives a thin zone of sharpness, so a subject can stand out against a soft background. A narrow aperture (a big f-number) keeps much more of the scene sharp from front to back. Distance matters too: the closer you are to your subject, and the longer your lens, the thinner the sharp zone becomes.
Which aperture should you choose?
- Portraits: f/1.8 to f/2.8 for a soft background, stopping down a little if you need both eyes sharp.
- Landscapes: f/8 to f/11 for sharpness across the scene.
- Street and travel: f/5.6 to f/8 gives a forgiving zone of sharpness while you move quickly.
- Low light: as wide as the lens allows, then raise the ISO.
Most lenses are sharpest one to two stops down from their widest opening. Very small apertures (f/16 and beyond) start to soften the whole picture through diffraction, so use the widest aperture that gives you the depth you need.