Aperture

The opening inside a lens that light passes through. Make it bigger and more light enters; smaller and less.

Why a smaller F-number means a bigger opening

The point that keeps tripping people up. The formula sorts it once and for all.

F-number = focal length ÷ aperture diameter

Aperture diameter is in the denominator, so:

• Diameter grows → F-number shrinks → f/1.4 (big opening = bright)
• Diameter shrinks → F-number grows → f/16 (small opening = dim)

In other words, the F-number is "the opening as a ratio to focal length." This is also why two lenses of different focal lengths give the same brightness at the same F-number.

Standard stops

Aperture moves in steps of √2 (about 1.414) because area must double for light to double.

f/1.4  f/2  f/2.8  f/4  f/5.6  f/8  f/11  f/16
←─────────────  open (bright, shallow DoF)
                                stopped down →─────  narrow (dim, deep DoF)

Side effect: depth of field

Aperture's real weapon is depth of field.

• Wide open (f/1.4): shallow DoF → subject sharp, background blurred (bokeh)
• Stopped down (f/8–f/11): deep DoF → everything sharp (landscapes, group shots)

DoF also depends on focal length, subject distance, and sensor size — but aperture is the one variable you control most directly while shooting.

Easy to confuse

• F-stop ≠ T-stop: F-stop is a geometric calculation; T-stop (transmission stop) measures actual light through the lens. Cinema lenses are marked in T-stops.
• Wider isn't always better: most lenses are sharpest stopped down 2–3 stops from wide open (the "sweet spot").

Related

• exposure-triangle — Parent note
• shutter-speed
• iso