Before light meters, photographers exposed by eye using the Sunny-16 rule. It still works: on a bright day, f/16 at one over your ISO gives a correct exposure. This calculator extends the rule across seven common lighting conditions and lists equivalent settings so you can shoot confidently without metering.
How it works
The base rule fixes the shutter at the reciprocal of the ISO and sets the aperture by the light:
Bright sun: f/16, shutter ≈ 1 ÷ ISO
Each cloudier condition opens the aperture one full stop, letting in twice the light to compensate for the dimmer scene:
| Condition | Aperture |
|---|---|
| Bright snow or sand | f/22 |
| Bright / hard sun | f/16 |
| Slight overcast | f/11 |
| Overcast | f/8 |
| Heavy overcast | f/5.6 |
| Open shade / sunset | f/4 |
| Golden hour | f/2.8 |
Once the base aperture and shutter are set, the tool holds the exposure value
constant (N² ÷ t) and lists every equivalent aperture and shutter pair, so you
can trade depth of field for motion control freely.
Why the rule works, and why it has lasted
The surface of the earth receives a remarkably consistent amount of sunlight on a clear day — it varies less than you might expect by latitude and season for typical outdoor shooting conditions. The Sunny-16 rule was codified from measured EV values and practical experience, and it encodes a light level that corresponds to the standard sunny day EV. Because the relationship between aperture, shutter speed, and ISO is mathematical, the rule expresses that EV as an easy-to-remember combination.
It has persisted because it is still accurate. Negative film and modern digital sensors have enough latitude that a half-stop error — which is about the worst the rule produces — is imperceptible. The rule also gives you the exposure relationship intuitively: if you know the f/16 base, you immediately know that overcast calls for f/8 (two stops open) and that a 500 ISO film shoots at roughly 1/500 s.
Adapting for light direction and subject tone
The standard Sunny-16 rule assumes front lighting — the sun behind the photographer’s shoulder, evenly lighting the subject’s face. Changing the light direction changes the effective exposure:
- Side lighting: the shadow side of the subject is one to two stops darker than metered. Open one stop or move your subject.
- Backlit subjects: the subject face is two to three stops darker than the background exposure. Open two stops for correct face exposure (and accept a blown-out background), or use fill flash.
- Very light subjects (pale skin, white clothing): the rule exposes well. Add half a stop for detail in white areas.
- Very dark subjects (dark skin, dark clothing): open half to one stop to prevent underexposure; the rule tends to give shadow detail at the expense of a slightly bright background.
Example and tips
At ISO 400 on a lightly overcast day, the rule gives f/11 at about 1/400 s. Want a blurred background instead? Open to f/2.8 and the equivalent list pushes the shutter to roughly 1/6400 s — same brightness, shallow focus.
Remember the rule assumes front lighting. Backlit or strongly side-lit subjects need one to two extra stops of exposure, and very reflective scenes like snow need one stop less. Treat Sunny-16 as a dependable starting point and bracket when the light is unusual.