Sunny 16 and its cousins: exposure with no meter at all
Before meters were cheap, every roll of film shipped with the answer folded inside the box. Kodak printed a little exposure table — sun, clouds, shade — and generations of photographers shot correctly exposed pictures by reading the sky instead of a needle. The whole system compresses into one sentence you can carry forever: in full sun, set the aperture to f/16 and the shutter to one over your film speed. ISO 100 film, f/16, 1/100th (or the nearest stop, 1/125). Done. That's sunny 16.
It works because the sun is astonishingly consistent. Midday sunlight on a clear day delivers close to the same illuminance everywhere on Earth — around EV 15 at ISO 100 — and film's latitude soaks up the remaining variation. The rule isn't folk wisdom; it's a photometric constant wearing a friendly name.
The cousins, by weather
Keep the shutter at one-over-ISO and walk the aperture with the sky:
f/16 — clean sun, hard-edged shadows. f/11 — slight haze, shadows going soft. f/8 — properly overcast, shadows barely there. f/5.6 — heavy grey blanket, no shadows at all. f/4 — open shade or sunset light.
Two refinements earn their keep. Snow and beaches bounce so much light that f/22 (or a faster shutter) saves the highlights — the rare case where the world is genuinely brighter than the rule assumes. And backlit subjects want two stops more than the sky suggests, because the rule meters the light, not the shaded face you're photographing.
Where the rule runs out
Indoors, immediately — interior light varies over a thousandfold range and no mnemonic survives that. Golden hour defeats it too: light drops a stop every few minutes near sunset, faster than any rule of thumb updates. And it says nothing about scenes the subject makes tricky — stage light, spotlit performers, a face by a window. Those are metering problems, not sky-reading problems, and they're where an actual reading (spot mode, ideally) pays for itself.
Why memorise it in the phone era
Three reasons, all practical. It's the sanity check: when any meter — this site's included — hands you a reading in daylight, you should know within a stop what it ought to say, and sunny 16 is how. It's the backup: batteries die, phones drown, and the rule doesn't. And it's the reference-match seed for this page's uncalibrated mode: told the device can't share exposure data, you can still point it at clean midday sun, type 15, tap match, and walk away with a calibrated meter — the rule bootstrapping the tool.
Kodak stopped printing the table decades ago. It kept working anyway; the sun didn't get a firmware update.