Aperture
The lens opening that admits light, expressed as f-number (e.g., f/1.4, f/2.8). Lower f-number = wider opening = more light + shallower depth of field.
The aperture is the iris inside a lens. f-number = focal length ÷ effective aperture diameter.
Stops
Each full stop doubles or halves light: f/1.4 → f/2 → f/2.8 → f/4 → f/5.6 → f/8 → f/11. One stop wider = double the light = halved shutter speed at the same ISO.
In smartphones
Fixed apertures (no iris) — typically f/1.6 to f/2.8 on the main camera. Wider apertures need larger lenses, which is why ultra-wide cameras tend toward f/2.2 and telephoto toward f/2.8.
Practical sweet spots
Most lenses are sharpest 1–2 stops down from wide open. An f/1.4 prime peaks around f/2.8–f/4. Use wide-open for shallow depth or low light; stop down for landscapes and group shots.