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Friday, October 28, 2011

Exposure Value

In photographyexposure value (EV) denotes all combinations of a camera's shutter speed and relative aperture that give the same exposure. In an attempt to simplify choosing among combinations of equivalent camera settings, the concept was developed by the German shutter manufacturer Friedrich Deckel (de) in the 1950s (Ray 2000, 318). 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, commonly referred to as a stop.[1]
Exposure value was originally indicated by the quantity symbol Ev; this symbol continues to be used in ISO standards, but the acronym EV is more common elsewhere.
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 light sensitive medium may exhibit reciprocity failure, which is a change of light sensitivity dependent on the intensity.



[edit]Formal definition

Exposure value is a base-2 logarithmic scale defined by (Ray 2000, 318)
\mathrm {EV} = \log_2 {\frac {N^2} {t} } \,,
EV 0 corresponds to an exposure time of 1 s and a relative aperture of f/1.0. If the EV is known, it can be used to select combinations of exposure time and f-number, as shown in Table 1.
Each increment of 1 in exposure value corresponds to a change of one “step” (or, more commonly, one “stop”) in exposure, i.e., half as much exposure, either by halving the exposure time or halving the aperture area, or a combination of such changes. Greater exposure values are appropriate for photography in more brightly lit situations, or for higher ISO speeds.

[edit]Camera settings vs. photometric exposure

“Exposure value” is somewhat of a misnomer, because it indicates combinations of camera settings rather than the photometric quantity of luminous exposure Hv (aka photometric exposure), which is given by (Ray 2000, 310)
H_\mathrm{v} = E_\mathrm{v} \cdot t \,,
The illuminance Ev is controlled by the f-number but also depends on the scene luminance Lv. To avoid confusion, some authors (Ray 2000, 310) have used camera exposure to refer to combinations of camera settings. The 1964 ASA standard for automatic exposure controls for cameras, ASA PH2.15-1964, took the same approach, and also used the more descriptive term camera exposure settings.
Common practice among photographers is nonetheless to use “exposure” to refer to camera settings as well as to photometric exposure.

[edit]EV as an indicator of camera settings

Table 1. Exposure times, in seconds or minutes (m), for various exposure values and f-numbers
−6602 m4 m8 m16 m32 m64 m128 m256 m512 m1024 m2048 m4096 m
−530602 m4 m8 m16 m32 m64 m128 m256 m512 m1024 m2048 m
−41530602 m4 m8 m16 m32 m64 m128 m256 m512 m1024 m
−381530602 m4 m8 m16 m32 m64 m128 m256 m512 m
−2481530602 m4 m8 m16 m32 m64 m128 m256 m
−12481530602 m4 m8 m16 m32 m64 m128 m
012481530602 m4 m8 m16 m32 m64 m
11/212481530602 m4 m8 m16 m32 m
21/41/212481530602 m4 m8 m16 m
31/81/41/212481530602 m4 m8 m
41/151/81/41/212481530602 m4 m
51/301/151/81/41/212481530602 m

[edit]Tabulated exposure values

An exposure meter may not always be available, and using a meter to determine exposure for some scenes with unusual lighting distribution may be difficult. However, natural light, as well as many scenes with artificial lighting, is predictable, so that exposure often can be determined with reasonable accuracy from tabulated values.
Table 2. Exposure values (ISO 100 speed) for various lighting conditions[3]
Lighting ConditionEV100
Light sand or snow in full or slightly hazy sunlight (distinct shadows)a16
Typical scene in full or slightly hazy sunlight (distinct shadows)a, b15
Typical scene in hazy sunlight (soft shadows)14
Typical scene, cloudy bright (no shadows)13
Typical scene, heavy overcast12
Areas in open shade, clear sunlight12
Outdoor, Natural light
Clear sky background15
Cloudy sky background14
Sunsets and skylines
Just before sunset12–14
At sunset12
Just after sunset9–11
The Moon,c altitude > 40°
Moonlight, Moon altitude > 40°
Full−3 to −2
Aurora borealis and australis
Bright−4 to −3
Medium−6 to −5
Outdoor, Artificial Light
Neon and other bright signs9–10
Night sports9
Fires and burning buildings9
Bright street scenes8
Night street scenes and window displays7–8
Night vehicle traffic5
Fairs and amusement parks7
Christmas tree lights4–5
Floodlit buildings, monuments, and fountains3–5
Distant views of lighted buildings2
Indoor, Artificial Light
Sports events, stage shows, and the like8–9
Circuses, floodlit8
Ice shows, floodlit9
Offices and work areas7–8
Home interiors5–7
Christmas tree lights4–5
  1. Values for direct sunlight apply between approximately two hours after sunrise and two hours before sunset, and assume front lighting. As a rough general rule, decrease EV by 1 for side lighting, and decrease EV by 2 for back lighting
  2. This is approximately the value given by the sunny 16 rule.
  3. These values are appropriate for pictures of the Moon taken at night with a long lens or telescope, and will render the Moon as a medium tone. They will not, in general, be suitable for landscape pictures that include the Moon. In a landscape photograph, the Moon typically is near the horizon, where its luminance changes considerably with altitude. Moreover, a landscape photograph usually must take account of the sky and foreground as well as the Moon. Consequently, it is nearly impossible to give a single correct exposure value for such a situation.
Exposure values in Table 2 are reasonable general guidelines, but they should be used with caution. For simplicity, they are rounded to the nearest integer, and they omit numerous considerations described in the ANSI exposure guides from which they are derived. Moreover, they take no account of color shifts or reciprocity failure. Proper use of tabluated exposure values is explained in detail in the ANSI exposure guide, ANSI PH2.7-1986.
The exposure values in Table 2 are for ISO 100 speed (“EV100”). For a different ISO speed S, increase the exposure values (decrease the exposures) by the number of exposure steps by which that speed is greater than ISO 100, formally
\mathrm{EV}_{S} = \mathrm{EV}_{100} + \log_2 \frac {S} {100} \,.
For example, ISO 400 speed is two steps greater than ISO 100:
\mathrm{EV}_{400} = \mathrm{EV}_{100} + \log_2 \frac {400} {100}
= \mathrm{EV}_{100} + 2 \,.
To photograph outdoor night sports with an ISO 400–speed imaging medium, search Table 2 for “Night sports” (which has an EV of 9), and add 2 to get EV400 = 11.
For lower ISO speed, decrease the exposure values (increase the exposures) by the number of exposure steps by which the speed is less than ISO 100. For example, ISO 50 speed is one step less than ISO 100:
\mathrm{EV}_{50} = \mathrm{EV}_{100} + \log_2 \frac {50} {100}
= \mathrm{EV}_{100} - 1 \,.
To photograph a rainbow against a cloudy sky with an ISO 50–speed imaging medium, search Table 2 for “Rainbows-Cloudy sky background” (which has an EV of 14), and subtract 1 to get EV50 = 13.

[edit]Setting EV on a camera

A Kodak Pony II camera with exposure value setting ring
On most cameras, there is no direct way to transfer an EV to camera settings; however, a few cameras, such as some Voigtländer and Braun models or the Kodak Pony II shown in the photo, allowed direct setting of exposure value.
Hasselblad Planar 80mm with EVS set at EV 12
Some medium-format cameras from Rollei (Rolleiflex,Rolleicord models) and Hasselblad allowed EV to be set on the lenses. 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. On some lenses the locking was optional, so that the photographer could chose the preferred method of working depending on the situation. Use of the EV scale on Hasselblad cameras is discussed briefly by Adams (1981, 39).

[edit]Exposure compensation in EV

Many current cameras allow for exposure compensation, and usually state it in terms of EV (Ray 2000, 316). In this context, EV refers to the difference between the indicated and set exposures. For example, an exposure compensation of +1 EV (or +1 step) means to increase exposure, by using either a longer exposure time or a smaller f-number.
The sense of exposure compensation is opposite that of the EV scale itself. An increase in exposure corresponds to a decrease in EV, so an exposure compensation of +1 EV results in a smaller EV; conversely, an exposure compensation of −1 EV results in a greater EV. For example, if a meter reading of a lighter-than-normal subject indicates EV 16, and an exposure compensation of +1 EV is applied to render the subject appropriately, the final camera settings will correspond to EV 15.

[edit]Meter indication in EV

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.

[edit]Relationship of EV to lighting conditions

The recommended f-number and exposure time for given lighting conditions and ISO speed are given by the exposure equation

\frac {N^2} {t} = \frac {L \cdot S} {K} \,,
Applied to the right-hand side of the exposure equation, exposure value is
\mathrm {EV} = \log_2 {\frac {L \cdot S} {K} } \,.
Camera settings also can be determined from incident-light measurements, for which the exposure equation is

\frac {N^2} {t} = \frac {E \cdot S} {C} \,,
  • E is the illuminance
  • C is the incident-light meter calibration constant
In terms of exposure value, the right-hand side becomes
\mathrm {EV} = \log_2 {\frac {E \cdot S} {C} } \,.
When applied to the left-hand side of the exposure equation, EV denotes actual combinations of camera settings; when applied to the right-hand side, EV denotes combinations of camera settings required to give the nominally “correct” exposure. The formal relationship of EV to luminance or illuminance has limitations. Although it usually works well for typical outdoor scenes in daylight, it is less applicable to scenes with highly atypical luminance distributions, such as city skylines at night. In such situations, the EV that will result in the best picture often is better determined by subjective evaluation of photographs than by formal consideration of luminance or illuminance.
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 or illuminance.

[edit]EV and APEX

The Additive system of Photographic EXposure (APEX) proposed in the 1960 ASA standard for monochrome film speed, ASA PH2.5-1960, extended the concept of exposure value to all quantities in the exposure equation by taking base-2 logarithms, reducing application of the equation to simple addition and subtraction. In terms of exposure value, the left-hand side of the exposure equation became
E_v = A_v + T_v \,,
where Av (aperture value) and Tv (time value) were defined as:
Av = log 2 A2
Tv = log 2 (1/T) \,,
  • A the relative aperture (f-number)
  • T the exposure time (“shutter speed”) in seconds[2]
Av and Tv represent the numbers of stops from f/1 and 1 second, respectively.
Use of APEX required logarithmic markings on aperture and shutter controls, however, and these never were incorporated in consumer cameras. With the inclusion of built-in exposure meters in most cameras shortly after APEX was proposed, the need to use the exposure equation was eliminated, and APEX saw little actual use.
Though it remains of little interest to the end user, APEX has seen a partial resurrection in the Exif standard, which calls for storing exposure data using APEX values. See Use of APEX values in Exif for additional discussion.

[edit]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 (CanonNikon, andSekonic[5]). Using K = 12.5, the relationship between EV at ISO 100 and luminance L is then
L = 2^{\mathrm {EV} - 3} \,.
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.[6]
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
E = 2.5 \times 2^{\mathrm {EV}} \,.
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  

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