Aperture and f-numbers
In short
- Aperture is the adjustable opening inside the lens.
- The f-number is a ratio: smaller number, bigger opening, more light.
- Wide apertures (f/1.8, f/2.8) blur the background; narrow ones (f/8, f/11) keep more in focus.
- Most lenses are sharpest about two stops down from their widest setting.
The opening inside the lens
Inside most lenses is a ring of thin blades that can close down to a smaller hole. This hole is the aperture. Opening it lets more light through; closing it lets less.
Why the numbers look backwards
The f-number is the lens's focal length divided by the diameter of the opening. On a 50 mm lens, an opening 25 mm wide is f/2; an opening about 6 mm wide is f/8.
Because it is a fraction, a bigger number means a smaller hole. It helps to read “f/8” as “one eighth”: one eighth is smaller than one half.
The standard full stops are f/1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. Each step halves the light. The numbers grow by about 1.4 (the square root of 2) because the area of a circle depends on the square of its width.
Aperture and depth of field
Aperture is the main tool for controlling how much of the scene is sharp. At f/1.8 a portrait subject can be sharp while the background melts into soft blur. At f/11 a landscape can be sharp from the flowers at your feet to the mountains.
Depth of field also depends on distance and focal length: getting closer or zooming in makes it shallower.
Choosing an aperture
Portraits and low light: wide, around f/1.8 to f/2.8.
Groups and street: middle, around f/5.6 to f/8.
Landscapes and architecture: f/8 to f/11.
Avoid the smallest settings (f/16 to f/22) unless you need them: light bends around the edges of a very small hole (diffraction) and the whole image gets slightly softer.