What exposure value is
Photographers use exposure value (EV) to describe how much light a combination of aperture and shutter speed lets in, as a single number:
EV = log₂(N² ÷ t)
where N is the f-number and t the shutter time in seconds. Each step of 1 EV is one stop: twice or half the light. Settings with the same EV give the same exposure, so f/8 at 1/125 s, f/5.6 at 1/250 s and f/11 at 1/60 s are all about EV 13.
Scene brightness and ISO
Light meters and exposure tables quote scene brightness as EV at ISO 100. If you use a higher ISO, the camera needs less light, so the scene it is exposing for is darker:
EV at ISO 100 = EV of the setting − log₂(ISO ÷ 100)
Worked example
You shoot at f/8, 1/125 s and ISO 100:
| Result | Value |
|---|---|
| EV of the setting | 13.0 |
| Scene brightness | EV 13, hazy sun to light overcast |
| Equivalent shutter at f/4 | 0.002 s (1/500 s) |
| Equivalent shutter at f/16 | 0.032 s (about 1/30 s) |
| Approximate illuminance | about 20,000 lux |
| Sunny 16 shutter at ISO 100 | 1/100 s |
The same settings at ISO 400 would correctly expose a scene two stops darker, EV 11 at ISO 100, like open shade.
Typical scene brightness (ISO 100)
| EV | Scene |
|---|---|
| 16 | Bright sun on snow or sand |
| 15 | Bright sunny day |
| 13 to 14 | Hazy sun, light overcast |
| 11 to 12 | Heavy overcast, open shade |
| 8 to 10 | Bright interior, sunset |
| 5 to 7 | Home interior at night |
| 1 to 4 | Night street, floodlit buildings at a distance |
| −2 to 0 | Moonlit landscape |
The sunny 16 rule
In bright sun, f/16 with a shutter speed of about 1 ÷ ISO gives a good exposure without a meter. From there, add one stop for slight haze, two for overcast and three for heavy overcast or open shade.
Tips
- Lux figures are rough: the conversion depends on the meter’s calibration constant.
- EV describes exposure only. Choose the aperture for depth of field and the shutter speed for motion, then adjust ISO to keep the EV.