Aperture explained: f-stops, depth of field and light

The short version: aperture is the size of the hole in the lens that lets light in. It’s written as an f-number. A small number like f/1.8 means a wide opening: more light and a blurrier background. A big number like f/11 means a narrow opening: less light and more of the scene in focus.

Why small numbers mean big openings

The f-number is a fraction. It’s the lens’s focal length divided by the width of the opening. On a 50mm lens, f/2 means an opening 25mm across, and f/8 means one only about 6mm across. That’s why the numbers seem backwards. f/2 is really “focal length over two”, which is far bigger than “focal length over eight”.

The f-stop scale

Each full step on this scale lets in half as much light as the one before:

f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22

Going from f/2.8 to f/4 halves the light, and the jump from f/4 to f/2.8 doubles it. Photographers call each of these jumps one stop. Most cameras also offer third-stops in between, such as f/3.2 and f/3.5.

What aperture does to your picture

Brightness. A wider aperture lets in more light, so you can use a faster shutter speed or a lower ISO. This matters indoors, at night and for fast action.

Depth of field. This is how much of the scene, front to back, looks sharp. Wide apertures give shallow depth of field: the subject is sharp and the background melts away. Narrow apertures keep more of the scene sharp.

Depth of field also gets shallower when you move closer to your subject or use a longer focal length. That’s why an 85mm lens at f/1.4 a couple of metres from someone gives that classic portrait blur.

Which aperture to use

Portraits: f/1.4 to f/2.8 for a blurred background. For two or more people at different distances, go to f/4 so everyone is sharp.
Landscapes: f/8 to f/11. Most lenses are at their sharpest around here, and you get plenty of depth of field.
Low light and indoor events: as wide as your lens goes, then stop down a little if faces aren’t sharp.
Street: f/5.6 to f/8 gives you room for error when focusing quickly.
Close-ups and macro: depth of field is tiny this close, so f/8 to f/16 is common even with a dedicated macro lens.

Don’t go too narrow

Past around f/11 to f/16, most lenses get slightly softer overall. This is called diffraction, and it happens because light bends as it squeezes through a very small opening. Use f/16 or f/22 when you really need the depth of field or a starburst effect on lights, not as your default.

Constant and variable aperture zooms

Some zooms keep the same maximum aperture all the way through the zoom range, such as a 24-70mm f/2.8 or 70-200mm f/4. Cheaper zooms are often listed like 18-55mm f/3.5-5.6, which means the widest aperture shrinks as you zoom in. A constant aperture zoom makes exposure simpler, especially for video.

Try this

Put your camera in aperture priority mode (A or Av), focus on something about a metre away with a background a few metres behind it, and take the same photo at your widest aperture, f/5.6 and f/11. The difference in background blur shows you more than any diagram.