By the Additive Photographic Exposure System, the exposure value is the sum of the Aperture Value and the Time Value:
When is the f-number:
When is the shutter time in seconds:
A light meter will read the amount of light from a scene giving a light value, and this will have to be translated to an EV depending on the film speed used, usually expressed as an ISO rating. For ISO 100 film, there is a one to one relationship. In practice, most light meters for photography will have a setting for film speed, and give exposure value or a list of suitable aperture/shutter combinations directly.
Some examples of EV to shutter/aperture combinations are:
| EV | Shutter | Aperture |
|---|---|---|
| 0 | 1 s | f/1.0 |
| 1 | 1 s | f/1.4 |
| 11 | 1/60 s | f/5.6 |
| 12 | 1/60 s | f/8 |
| 12 | 1/125 s | f/5.6 |
| 13 | 1/125 s | f/8 |
| 13 | 1/250 s | f/5.6 |
| 14 | 1/250 s | f/8 |
| 19 | 1/2000 s | f/16 |
Some cameras, like the Hasselblad, have markings for EV.
See also: light value