The Sunny 16 rule works because the sun is the most consistent light source you will ever shoot under. From mid-morning to mid-afternoon on a clear day, the light hitting a scene is close enough to constant that one exposure — f/16 at a shutter speed of 1/ISO — lands correctly almost anywhere on Earth.

This matters more than it first appears, because your camera's meter does something different. A meter reads light *reflected* off the scene and assumes everything averages out to a middle grey. Point it at a dark doorway and it brightens the shot; point it at a bright sky and it darkens your subject into a smudge. Sunny 16 ignores all of that. It exposes for the light *falling on* the scene — so a dark subject stays dark, a bright one stays bright, and shadows stay genuinely black instead of being dragged up to grey.

The base setting is also just a starting point. f/16 at 1/125 on ISO 100 is the same exposure as f/11 at 1/250, or f/8 at 1/500. Trade aperture for shutter speed freely: same light, different look. Most street photographers take the f/8 or f/11 equivalent for a faster shutter and a workable zone-focus depth of field.

You do not need to judge how cloudy it is. Look at the shadows on the ground — they tell you the aperture. Shutter speed stays at 1/ISO (or the next fastest available) throughout.

Sunny 16 is older than the light meter in your camera. Before through-the-lens metering became standard in the 1960s and 70s, film manufacturers printed daylight exposure tables on the paper insert inside every box of film — Kodak called theirs a 'basic daylight exposure' guide. The tables said essentially what this card says: f/16 in bright sun at a shutter speed matching your film speed, then open up as the light softens.

Handheld meters existed, but they were slow, fragile and one more thing to carry. Working photographers memorised the table instead, and it stuck because it kept working. Most of the classic street photography you have seen was made on cameras with no built-in meter at all — early Leicas among them — by photographers who set exposure from the light and then forgot about it. Garry Winogrand worked pre-set and fast for exactly this reason: the picture on the street does not wait while you meter.

Your eye stays on the scene. The moment you look at a meter readout, you have stopped watching the street. With exposure pre-set, the camera comes up, the frame happens, the camera comes down.

High-contrast scenes stop fooling you. The classic street situation — a subject stepping through a shaft of light between dark buildings — is exactly where reflected meters fail. They see all that shadow and overexpose, blowing your highlights and lifting blacks to mud. Sunny 16 exposes for the lit subject and lets the shadows fall to black, which is usually the picture you wanted.

It pairs with zone focusing. Pre-set exposure plus pre-set focus turns any camera into a point-and-shoot with your judgement behind it. Nothing to change, nothing to hunt, no lag.

Your edit gets easier. Frames shot in the same light at the same settings sit together consistently. No exposure lottery from the meter re-deciding every frame.

Trusting it at golden hour. The rule assumes the sun is well up. In the last hour before sunset the light falls off fast — roughly a stop every ten to fifteen minutes — and Sunny 16 drifts from accurate to underexposed. Open up progressively, or switch back to the meter as the sun drops.

Forgetting the jump into shade. Sun to open shade is about a two-stop change. Walking from a lit pavement into a shaded arcade needs f/16 to become f/8. The habit to build: every time the light on the ground changes, your thumb moves before your eye does.

Not deciding on backlight. A face lit from behind needs one and a half to two stops more than the rule gives — or none at all, if you want the silhouette. Either is fine; the mistake is not choosing.

Expecting slide-film precision. On digital, shoot raw and a half-stop miss is nothing. The rule gets you within the latitude of the file; it does not need to be perfect, it needs to be fast.