By the Additive Photographic Exposure System, the exposure value is the sum of the Aperture Value and the Time Value:
When <math>N</math> is the f-number:
When <math>t</math> 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
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