Aperture (f-stop)

The size of the opening that light passes through inside the lens. The smaller the f-number (f/1.4), the bigger the opening, so the sharp range (depth of field) is shallow and the background blurs a lot. The larger the f-number (f/16), the smaller the opening, so front and back are both sharp. It is the dial that controls deep focus, shallow depth of field and bokeh. In AI prompts, official guides suggest writing the result, like "shallow depth of field", along with the f-number rather than the number alone.

Camera position seen from outside. From the same spot and with the same lens as shallow depth of field, the blue camera frames the bunny from the waist up. The camera does not move; only the aperture changes.

Camera view. Over 8 seconds the aperture opens to around f/1.4, closes to around f/16 and opens again, so the blur on the buildings behind grows and shrinks. In the comparison it stays at f/16 and even the buildings are sharp.

How it feels

Control
Focus
Separation
Realism

When to use it

Blowing out the background so only the person stands out (f/1.4-f/2)
Making bokeh big and soft
Needing deep focus with both front and back sharp (f/11-f/16)

In the movies

Barry Lyndon (1975) · Source: Criterion Collection (YouTube) · Original video

Citizen Kane 1941
Cinematographer Gregg Toland wrote that instead of the f/2.3-f/2.8 common at the time, he shot most interiors at f/8 or smaller, some at f/11, and one scene at f/16.

Not to be confused with

Focal Length: The mm on a lens (focal length) sets how wide a view it takes in; the f-number sets depth of field and brightness. Both are printed on the lens, so they often get mixed up.
Shallow Depth of Field: Shallow depth of field is the result; opening the aperture is one way to get it.
Deep Focus: Deep focus is the result; stopping down the aperture is one way to get it.

Prompt

Keyword

wide open aperture

Descriptive

Wide open aperture, f/1.4, shallow depth of field. The subject is sharp from the waist up while the buildings behind melt into soft blur.

Try these combinations

f/1.4 + portrait: a heavily blurred background
Portrait shot at f/1.4, wide open aperture, shallow depth of field, soft creamy bokeh behind the subject
f/11 + wide lens: sharp front to back
Wide-angle street scene stopped down to f/11, deep focus, everything from the foreground to the far buildings in sharp focus