luxmetered the rule that shipped in every film box

Sunny 16,
on dials.

Before meters were cheap, the exposure table came folded in the film box. This is that table with knobs on: your ISO and your sky in, camera settings out — plus every equivalent pair on the aperture ring. No camera, no reading, just the rule.

The rule, dialled in

film / sensor ISO

the sky above you

—

shutter = one over ISO, snapped to the nearest real stop · backlit subjects want two stops more than the sky suggests

Same light, other dials

equivalent exposures for your ISO and sky — pick the pair your subject wants

aperture	shutter	suits

How far to trust it

midday, outdoors — from about two hours after sunrise to two before sunset, the sun is a photometric constant; that is the rule's whole kingdom
golden hour breaks it — light falls a stop every few minutes near the horizon; switch to an actual reading and re-meter often
indoors it is useless — interior light spans a thousandfold range and no mnemonic survives that
film forgives, slides don't — negative film's latitude absorbs the rule's rounding; slide film deserves a meter even at noon
long exposures change the maths — past a second on film, reciprocity failure wants extra time; the reciprocity page does that arithmetic

Where the rule comes from

Full sun at midday delivers about EV 15 at ISO 100 — the calculator prints the exact figure for your sky so you can cross-check it against the meter. The long version, with the history and the failure modes, is in the darkroom essay Sunny 16 and its cousins.

← back to the light meter — for every scene the sky won't tell you about.