What does the f-number on a lens mean?
The aperture — how wide the lens opening is relative to its focal length — and the scale is counterintuitive in two ways that confuse almost everyone at first.
The two confusions:
Lower numbers mean wider apertures. f/1.8 is a much larger opening than f/16. This is because the number is a ratio: focal length divided by aperture diameter. A bigger denominator gives a smaller number.
The steps are not linear. The standard sequence — f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 — each halves the light admitted. The numbers progress by a factor of roughly 1.4 because light gathering depends on area, which scales with the square of the diameter.
What aperture controls, and this is why it matters:
Light. A wider aperture admits more, allowing faster shutter speeds or lower sensitivity in poor conditions.
Depth of field — how much of the scene is in acceptable focus. Wide apertures produce shallow depth of field, isolating a subject against a blurred background. Narrow apertures keep more in focus, which is what landscape photography wants.
These two are linked and cannot be separated, which is the central trade-off in exposure.
The exposure triangle. Aperture, shutter speed and ISO together determine exposure. Changing one requires compensating with another, and each has a side effect: aperture affects depth of field, shutter speed affects motion blur, ISO affects noise.
Why phone apertures are usually fixed. Most phone cameras have no adjustable aperture at all — a single fixed value, typically wide. Depth of field is then simulated in software rather than produced optically.
Why a "fast" lens is expensive. A wide maximum aperture requires larger glass elements, precisely made, and cost rises sharply.
Diffraction limits the other end: stopping down too far softens the image, so the sharpest aperture is usually a couple of stops from the widest rather than the narrowest.
T-stops measure actual transmitted light and are used in cinematography.