Exposure value

Encyclopedia : E : EX : EXP : Exposure value

Fast shutter speed, short exposure

Slow Shutter speed, long exposure

In photography, exposure value (EV) is a value given to all combinations of camera shutter speed and aperture that gives the same exposure. By definition, EV 0 corresponds to the total exposure of a shutter speed of 1 s and an aperture of f/1.0. It is a base 2 negative logarithmic scale, where an increase of one EV corresponds to half the amount of light admitted.

By the Additive Photographic Exposure System (APEX), the exposure value is the sum of the Aperture Value and the Time Value:

[\mathit = \mathit + \mathit]

If N is the f-number (such as 2.8, 4.0, 5.6, ...), the Aperture Value is

[\mathit = \log_2 N^2 = 2\,\log_2 N].

If t is the shutter time in seconds, the Time Value is

[\mathit = \log_2 \frac1t = -\log_2 t].

A light meter will read the amount of light from a scene giving a brightness value (BV), and this will have to be translated to an EV depending on the film speed used, usually expressed as an ISO rating or a speed value (SV), using EV = BV + SV.

Some meters indicate a light value that, for ISO 100 film, is equal to the exposure value required for a "proper" exposure. Slower films with ISO films speeds below 100 require a corresponding increase in exposures, i.e. lower exposure values. 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 cameras, like certain Hasselblad models, also have markings for EV.

Shutter/aperture combinations and corresponding exposure values are listed in the following table:

**Table of exposure values (ISO 100)**
	f-number
shutter (s)	1.0	1.4	2.0	2.8	4.0	5.6	8.0	11	16	22	32	45	64
60	−6	−5	−4	−3	−2	−1	0	1	2	3	4	5	6
30	−5	−4	−3	−2	−1	0	1	2	3	4	5	6	7
15	−4	−3	−2	−1	0	1	2	3	4	5	6	7	8
8	−3	−2	−1	0	1	2	3	4	5	6	7	8	9
4	−2	−1	0	1	2	3	4	5	6	7	8	9	10
2	−1	0	1	2	3	4	5	6	7	8	9	10	11
1	0	1	2	3	4	5	6	7	8	9	10	11	12
1/2	1	2	3	4	5	6	7	8	9	10	11	12	13
1/4	2	3	4	5	6	7	8	9	10	11	12	13	14
1/8	3	4	5	6	7	8	9	10	11	12	13	14	15
1/15	4	5	6	7	8	9	10	11	12	13	14	15	16
1/30	5	6	7	8	9	10	11	12	13	14	15	16	17
1/60	6	7	8	9	10	11	12	13	14	15	16	17	18
1/125	7	8	9	10	11	12	13	14	15	16	17	18	19
1/250	8	9	10	11	12	13	14	15	16	17	18	19	20
1/500	9	10	11	12	13	14	15	16	17	18	19	20	21
1/1000	10	11	12	13	14	15	16	17	18	19	20	21	22
1/2000	11	12	13	14	15	16	17	18	19	20	21	22	23
1/4000	12	13	14	15	16	17	18	19	20	21	22	23	24
1/8000	13	14	15	16	17	18	19	20	21	22	23	24	25

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