Exposure Value - EV As A Measure of Luminance and Illuminance

EV As A Measure of Luminance and Illuminance

For a given ISO speed and meter calibration constant, there is a direct relationship between exposure value and luminance (or illuminance). Strictly, EV is not a measure of luminance or illuminance; rather, an EV corresponds to a luminance (or illuminance) for which a camera with a given ISO speed would use the indicated EV to obtain the nominally correct exposure. Nonetheless, it is common practice among photographic equipment manufacturers to express luminance in EV for ISO 100 speed, as when specifying metering range (Ray 2000, 318) or autofocus sensitivity. And the practice is long established; Ray (2002), 592) cites Ulffers (1968) as an early example. Properly, the meter calibration constant as well as the ISO speed should be stated, but this seldom is done.

Values for the reflected-light calibration constant K vary slightly among manufacturers; a common choice is 12.5 (Canon, Nikon, and Sekonic). Using K = 12.5, the relationship between EV at ISO 100 and luminance L is then

Values of luminance at various values of EV based on this relationship are shown in Table 3. Using this relationship, a reflected-light exposure meter that indicates in EV can be used to determine luminance.

As with luminance, common practice among photographic equipment manufacturers is to express illuminance in EV for ISO 100 speed when specifying metering range.

The situation with incident-light meters is more complicated than that for reflected-light meters, because the calibration constant C depends on the sensor type. Two sensor types are common: flat (cosine-responding) and hemispherical (cardioid-responding). Illuminance is measured with a flat sensor; a typical value for C is 250 with illuminance in lux. Using C = 250, the relationship between EV at ISO 100 and illuminance E is then

Values of illuminance at various values of EV based on this relationship are shown in Table 3. Using this relationship, an incident-light exposure meter that indicates in EV can be used to determine illuminance.

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, typical values for C are between 320 (Minolta) and 340 (Sekonic) with illuminance in lux. If illuminance is interpreted loosely, measurements with a hemispherical sensor indicate “scene illuminance”.

Exposure meter calibration is discussed in detail in the Light meter article.

Table 3. Exposure value vs. luminance (ISO 100, K = 12.5) and illuminance (ISO 100, C = 250)

EV100	Luminance		Illuminance
	cd/m2	fL	lx	fc
−4	0.008	0.0023	0.156	0.015
−3	0.016	0.0046	0.313	0.029
−2	0.031	0.0091	0.625	0.058
−1	0.063	0.018	1.25	0.116
0	0.125	0.036	2.5	0.232
1	0.25	0.073	5	0.465
2	0.5	0.146	10	0.929
3	1	0.292	20	1.86
4	2	0.584	40	3.72
5	4	1.17	80	7.43
6	8	2.33	160	14.9
7	16	4.67	320	29.7
8	32	9.34	640	59.5
9	64	18.7	1280	119
10	128	37.4	2560	238
11	256	74.7	5120	476
12	512	149	10,240	951
13	1024	299	20,480	1903
14	2048	598	40,960	3805
15	4096	1195	81,920	7611
16	8192	2391	163,840	15,221