Exposure value

In photography, exposure value (EV) denotes all combinations of camera shutter speed and relative aperture that give the same exposure. Exposure value also is used to indicate an interval on the photographic exposure scale, with 1 EV corresponding to a standard power-of-2 exposure step.^[1]

Exposure value originally was indicated by the quantity symbol $E_{v}$ ; this symbol continues to be used in ISO standards, but the acronym EV now is more common elsewhere.

Introduction

Although all camera settings with the same exposure value nominally give the same exposure, they do not necessarily give the same picture. The exposure time (“shutter speed”) determines the amount of motion blur, as illustrated by the two images at the right, and the relative aperture determines the depth of field.

The concept of exposure value was developed in Germany in the 1950s (Ray 2000), in attempt to simplify application of the camera exposure equation

{\frac {N^{2}}{t}}={\frac {LS}{K}}

where

$N$ is the relative aperture (f-number)
$t$ is the exposure time (“shutter speed”)
$L$ is the average scene luminance
$S$ is the ISO linear speed
$K$ is the reflected-light meter calibration constant

Symbols for the quantities above have varied over time; ASA PH 2.5-1960, which proposed the Additive system of Photographic EXposure (APEX), used $A$ , $T$ , $B$ , and $S_{x}$ for relative aperture, exposure time, luminance, and arithmetic film speed. ISO 2720-1974 uses $A$ for relative aperture. The symbols used above reflect current practice for many authors, such as Ray (2000).

Exposure value is a base-2 logarithmic scale defined by

\mathrm {EV} =\log _{2}{\frac {N^{2}}{t}}=\log _{2}{\frac {LS}{K}}

APEX extended the concept of exposure value to incident-light measurements, for which the exposure equation is

{\frac {N^{2}}{t}}={\frac {ES}{C}}

where

$E$ is the illuminance
$C$ is the incident-light meter calibration constant

In terms of exposure value, this becomes

\mathrm {EV} =\log _{2}{\frac {N^{2}}{t}}=\log _{2}{\frac {ES}{C}}

Exposure value has two interpretations: when applied to the left-hand side of the exposure equation, EV denotes combinations of camera settings; when applied to the right-hand side, EV denotes combinations of luminance (or illuminance) and film speed. The former interpretation is the EV that actually is set, while the latter is the EV that is required to give the nominally “correct” exposure.

With the former interpretation, EV 0 corresponds to an exposure time of 1 s and a relative aperture of $f$ /1.0. Other combinations of exposure time and relative aperture, with corresponding exposure values, are shown in Table 1.

Note that for a given luminance and film speed, a greater EV results in less exposure, and for fixed exposure (i.e., fixed camera settings), a greater EV corresponds to greater luminance.

EV as an indicator of camera settings

Table 1. Exposure Values for Various Shutter Times and f-numbers

shutter (s)	1.0	1.4	2.0	2.8	4.0	5.6	8.0	11	16	22	32	45	64
	`f`-number
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

EV as a measure of luminance

For a given ISO speed, exposure value can be used as a measure of luminance. Common practice among photographic equipment manufacturers is to express luminance in EV for ISO 100 speed (e.g., when specifying metering range or autofocus sensitivity). The relationship between EV and luminance also depends on the calibration constant $K$ , which often is not stated. The recommended values for $K$ have changed slightly over the years, and the applicable ANSI and ISO standards usually have recommended a range of values. The current standard, ISO 2720-1974, recommends a range of 10.6 to 13.4 with luminance in cd/m². In practice, there is far less variation. Two values for $K$ are in common use: 12.5 (Canon, Nikon, and Sekonic^[2]) and 14 (Minolta and Pentax); the difference between the two values is approximately 1/6 EV. The relationship between EV at ISO 100 and luminance is particularly simple when $K$ = 12.5; then

L=2^{\mathrm {EV} -3}

Values of luminance at various values of EV using this relationship are shown in Table 2.

Exposure value simplifies determination of a scene's luminance range, as well as adjustment of exposure (“exposure compensation”) relative to the recommended value, because a change of 1 EV corresponds to a standard power-of-2 exposure step, i.e., either a halving or doubling of exposure.

Table 2. Exposure Value vs. Luminance (ISO 100, K = 12.5)

EV	`L`, cd/m²
0	0.125
1	0.25
2	0.5
3	1
4	2
5	4
6	8
7	16
8	32
9	64
10	128
11	256
12	512
13	1024
14	2048
15	4096
16	8192

EV as a measure of illuminance

For a given ISO speed, EV can be interpreted as a measure of illuminance. As with luminance, common practice among photographic equipment manufacturers is to express illuminance in EV for ISO 100 speed (e.g., when specifying metering range^[3]).

The relationship between EV and illuminance also depends on the calibration constant $C$ . The situation is more complicated than that for reflected-light meters, because the calibration constant depends on the sensor type. Two sensor types are common: flat (cosine-responding) and hemispherical (cardioid-responding); illuminance is measured with a flat sensor. ISO 2720-1974 recommends a range for $C$ of 240 to 400 with illuminance in lux; a value of 250 is commonly used. The relationship between EV at ISO 100 and luminance then is

E=2.5\cdot 2^{\mathrm {EV} }

Values of illuminance at various values of EV using this relationship are shown in Table 3.

Table 3. Exposure Value vs. Illuminance (ISO 100, C = 250)

EV	`E`, lx
0	2.5
1	5
2	10
3	20
4	40
5	80
6	160
7	320
8	640
9	1280
10	2560
11	5120
12	10240
13	20480
14	40960
15	81920
16	163840

Although illuminance measurements may indicate appropriate exposure for a flat subject, they are less useful for a typical scene in which many elements are not flat and are at various orientations to the camera. For determining practical photographic exposure, a hemispherical sensor has proven more effective. With a hemispherical sensor, ISO 2720-1974 recommends a range for $C$ of 320 to 540 with illuminance in lux; in practice, values typically are between 320 (Minolta) and 340 (Sekonic). The relative responses of flat and hemispherical sensors depend upon the number and type of light sources; when each sensor is pointed at a small light source,^[4] a hemispherical sensor with $C=330$ will indicate an exposure approximately 0.40 step greater than that indicated by a flat sensor with $C=250$ . If illuminance is interpreted loosely, measurements with a hemispherical sensor indicate “scene illuminance.”

Setting EV on a camera

In most cases there is no direct way to transfer an EV to camera settings; however, there have been a few notable exceptions, such as some Hasselblad lenses introduced In the late 1950s that allowed EV to be set directly. The set EV could be locked, coupling shutter and aperture settings, such that adjusting either the shutter speed or aperture made a corresponding adjustment in the other to maintain a constant exposure. Use of the EV scale on such cameras is discussed briefly by Adams (1981, 39).

Meter indication in EV

Calibration of cameras with internal meters is covered by ISO 2721-1982; nonetheless, many manufacturers continue to specify (though seldom state) exposure calibration in terms of $K$ , and many calibration instruments (e.g., Kyoritsu-Arrowin multi-function camera testers^[5] ) use the specified $K$ to set the test parameters.

Some light meters (e.g., Pentax spot meters) indicate directly in EV at ISO 100. Some other meters, especially digital models, can indicate EV for the selected ISO speed. In most cases, this difference is irrelevant; with the Pentax meters, camera settings usually are determined using the exposure calculator, and most digital meters directly display shutter speeds and $f$ -numbers.

Recently, articles on many web sites have used light value (LV) to denote EV at ISO 100. However, this term does not derive from a standards body, and has had several conflicting definitions.

EV and APEX

The Additive system of Photographic EXposure (APEX) was an attempt to extend the concept of exposure value to all exposure parameters and simplify exposure computation. However, the use of APEX required logarithmic aperture and shutter markings, and these controls were never incorporated in consumer cameras. With the inclusion of exposure meters in many small-format in the late 1960s, the need to compute exposures was eliminated, and APEX saw little actual use.

Notes

^ Here, "step" refers to a division of the exposure scale. It is common for photographers and photographic literature to refer to such a step as "one stop," although properly, a stop is a device that controls the amount of light that passes through a lens. Some authors, e.g., Davis (1999), prefer the term "stop" because they refer to steps (e.g., on a step tablet) that are other than powers of 2. ISO standards generally use “step,” while “stop” is by far the most common elsewhere. The use of $E_{v}$ or EV to refer to a power-of-2 step would seem to avoid the ambiguity.
^ Specifications for Sekonic light meters are available on the Sekonic web site under “Products.”
^ The metering range for an incident-light meter specified in EV at ISO 100 usually applies to a hemispherical sensor, so strictly speaking, it is not a measure of illuminance
^ ISO 2720-1974 calls for incident-light calibration to be measured by aiming the sensor at a point source in a darkened room.
^ Specifications for Kyoritsu testers are available on the C.R.I.S. Camera Services web site under “kyoritsu test equipment.”

References

Adams, Ansel. 1981. The Negative. Boston: New York Graphic Society.

ASA PH2.5-1960. American Standard Method for Determining Speed of photographic Negative Materials (Monochrome, Continuous Tone). New York: United States of America Standards Institute

ANSI/ISO 2721-1974 (R1994). General Purpose Photographic Exposure Meters (Photoelectric Type)—Guide to Product Specification. New York: American National Standards Institute.

ISO 2721-1982. Photography — Cameras — Automatic controls of exposure. International Organization for Standardization.

Davis, Phil. 1999. Beyond the Zone System, 4th ed. Boston: Focal Press.

Ray, Sidney F. 2000. “Camera Exposure Determination.” In The Manual of Photography, 9th ed. Oxford: Focal Press.

External links

[1] Here, "step" refers to a division of the exposure scale. It is common for photographers and photographic literature to refer to such a step as "one stop," although properly, a stop is a device that controls the amount of light that passes through a lens. Some authors, e.g., Davis (1999), prefer the term "stop" because they refer to steps (e.g., on a step tablet) that are other than powers of 2. ISO standards generally use “step,” while “stop” is by far the most common elsewhere. The use of $E_{v}$ or EV to refer to a power-of-2 step would seem to avoid the ambiguity.

[2] Specifications for Sekonic light meters are available on the Sekonic web site under “Products.”

[3] The metering range for an incident-light meter specified in EV at ISO 100 usually applies to a hemispherical sensor, so strictly speaking, it is not a measure of illuminance

[4] ISO 2720-1974 calls for incident-light calibration to be measured by aiming the sensor at a point source in a darkened room.

[5] Specifications for Kyoritsu testers are available on the C.R.I.S. Camera Services web site under “kyoritsu test equipment.”

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