The iris and what it does
The aperture diaphragm sits inside the lens, a ring of overlapping metal leaves that form an adjustable hole. Make it smaller — stop down — and three things happen at once: less light reaches the film or sensor, the exposure time must lengthen or the ISO rise to compensate, and the zone of acceptable focus deepens.
That last effect is depth of field, and it is the reason stopping down exists as a creative choice rather than a mere exposure correction. At f/2 a portrait lens renders the plane of sharp focus in millimetres; at f/11 the same lens may hold sharpness from a metre to infinity. Landscape photographers from Gustave Le Gray onward understood this — a stopped-down lens could hold both foreground rocks and a distant horizon in a single frame, something an open aperture simply cannot do.
The unit is the f-stop: the focal length divided by the aperture diameter, so f/8 on a 50 mm lens means a 6.25 mm opening. Because the relationship is a ratio, f/8 delivers the same exposure regardless of focal length — a property that makes the f-number universal across every format and every era of the medium.
Each full stop halves the light: f/5.6 admits twice the light of f/8, half the light of f/4. The sequence runs f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 — each step a factor of √2, which keeps the area of the circular opening exactly double or half its neighbour.
There is a ceiling to the benefit. Stop down past a lens's sweet spot — often somewhere around f/8 to f/11 on a standard lens — and diffraction softens the image faster than depth of field improves it. The wave nature of light imposes a limit that geometry alone cannot overcome.
The whole of practical exposure is a negotiation between these numbers: aperture, shutter speed, and sensitivity, each one moving the others. Aperture is where that negotiation usually starts.