Exposure value
Encyclopedia
In photography
, exposure 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 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.
Exposure value was originally indicated by the quantity symbol
; 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.
ic scale defined by (Ray 2000, 318)
where
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 speed
s.
quantity of luminous exposure Hv (aka photometric exposure), which is given by (Ray 2000, 310)
where
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.
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.
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
,
increase the exposure values (decrease the exposures) by the number of exposure steps by which that speed is greater than
ISO 100, formally
For example, ISO 400 speed is two steps greater than ISO 100:
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
.
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:
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
.
and Braun
models or the Kodak Pony II shown in the photo, allowed direct setting of exposure value.
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).
, 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
-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.
s (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
-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.
and ISO speed are given by the exposure equation
where
Applied to the right-hand side of the exposure equation, exposure value is
Camera settings also can be determined from incident-light measurements,
for which the exposure equation is
where
In terms of exposure value, the right-hand side becomes
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.
)
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
where Av (aperture value) and Tv (time value) were defined as:
and
with
Av and Tv represent the numbers of stops from 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.
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 , 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 ,
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.
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
, exposure value (EV) denotes all combinations of a camera
Camera
A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...
's shutter speed
Shutter speed
In photography, shutter speed is a common term used to discuss exposure time, the effective length of time a camera's shutter is open....
and relative aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
that give the same exposure
Exposure (photography)
In photography, exposure is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value and scene luminance over a specified area.In photographic jargon, an exposure...
. In an attempt to simplify choosing among combinations of equivalent camera settings, the concept was developed by the German shutter manufacturer 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.
Exposure value was originally indicated by the quantity symbol
; 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
Shutter speed
In photography, shutter speed is a common term used to discuss exposure time, the effective length of time a camera's shutter is open....
”) determines the amount of motion blur
Motion blur
Motion blur is the apparent streaking of rapidly moving objects in a still image or a sequence of images such as a movie or animation. It results when the image being recorded changes during the recording of a single frame, either due to rapid movement or long exposure.- Photography :When a camera...
, as illustrated by the two images at the right, and the relative aperture determines the depth of field
Depth of field
In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...
. The light sensitive medium may exhibit reciprocity failure, which is a change of light sensitivity dependent on the intensity.
Formal definition
Exposure value is a base-2 logarithmLogarithm
The logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...
ic scale defined by (Ray 2000, 318)
where
- N is the relative apertureApertureIn optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
(f-numberF-numberIn optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...
) - t is the exposure time (“shutter speedShutter speedIn photography, shutter speed is a common term used to discuss exposure time, the effective length of time a camera's shutter is open....
”) in seconds
EV 0 corresponds to an exposure time of 1 s
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
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 speed
Film speed
Film speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system....
s.
Camera settings vs. photometric exposure
“Exposure value” is somewhat of a misnomer, because it indicates combinations of camera settings rather than the photometricPhotometry (optics)
Photometry is the science of the measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of radiant energy in terms of absolute power; rather, in photometry, the radiant power at each wavelength is weighted by...
quantity of luminous exposure Hv (aka photometric exposure), which is given by (Ray 2000, 310)
where
- Hv is the photometric / luminous exposure
- Ev is the image-plane illuminanceIlluminanceIn photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per...
- t is the exposure time
The illuminance Ev is controlled by the f-number but also depends on the scene luminance
Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
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.
EV as an indicator of camera settings
| EV | f-number | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1.0 | 1.4 | 2.0 | 2.8 | 4.0 | 5.6 | 8.0 | 11 | 16 | 22 | 32 | 45 | 64 | |
| −6 | 60 | 2 m | 4 m | 8 m | 16 m | 32 m | 64 m | 128 m | 256 m | 512 m | 1024 m | 2048 m | 4096 m |
| −5 | 30 | 60 | 2 m | 4 m | 8 m | 16 m | 32 m | 64 m | 128 m | 256 m | 512 m | 1024 m | 2048 m |
| −4 | 15 | 30 | 60 | 2 m | 4 m | 8 m | 16 m | 32 m | 64 m | 128 m | 256 m | 512 m | 1024 m |
| −3 | 8 | 15 | 30 | 60 | 2 m | 4 m | 8 m | 16 m | 32 m | 64 m | 128 m | 256 m | 512 m |
| −2 | 4 | 8 | 15 | 30 | 60 | 2 m | 4 m | 8 m | 16 m | 32 m | 64 m | 128 m | 256 m |
| −1 | 2 | 4 | 8 | 15 | 30 | 60 | 2 m | 4 m | 8 m | 16 m | 32 m | 64 m | 128 m |
| 0 | 1 | 2 | 4 | 8 | 15 | 30 | 60 | 2 m | 4 m | 8 m | 16 m | 32 m | 64 m |
| 1 | 1/2 | 1 | 2 | 4 | 8 | 15 | 30 | 60 | 2 m | 4 m | 8 m | 16 m | 32 m |
| 2 | 1/4 | 1/2 | 1 | 2 | 4 | 8 | 15 | 30 | 60 | 2 m | 4 m | 8 m | 16 m |
| 3 | 1/8 | 1/4 | 1/2 | 1 | 2 | 4 | 8 | 15 | 30 | 60 | 2 m | 4 m | 8 m |
| 4 | 1/15 | 1/8 | 1/4 | 1/2 | 1 | 2 | 4 | 8 | 15 | 30 | 60 | 2 m | 4 m |
| 5 | 1/30 | 1/15 | 1/8 | 1/4 | 1/2 | 1 | 2 | 4 | 8 | 15 | 30 | 60 | 2 m |
| 6 | 1/60 | 1/30 | 1/15 | 1/8 | 1/4 | 1/2 | 1 | 2 | 4 | 8 | 15 | 30 | 60 |
| 7 | 1/125 | 1/60 | 1/30 | 1/15 | 1/8 | 1/4 | 1/2 | 1 | 2 | 4 | 8 | 15 | 30 |
| 8 | 1/250 | 1/125 | 1/60 | 1/30 | 1/15 | 1/8 | 1/4 | 1/2 | 1 | 2 | 4 | 8 | 15 |
| 9 | 1/500 | 1/250 | 1/125 | 1/60 | 1/30 | 1/15 | 1/8 | 1/4 | 1/2 | 1 | 2 | 4 | 8 |
| 10 | 1/1000 | 1/500 | 1/250 | 1/125 | 1/60 | 1/30 | 1/15 | 1/8 | 1/4 | 1/2 | 1 | 2 | 4 |
| 11 | 1/2000 | 1/1000 | 1/500 | 1/250 | 1/125 | 1/60 | 1/30 | 1/15 | 1/8 | 1/4 | 1/2 | 1 | 2 |
| 12 | 1/4000 | 1/2000 | 1/1000 | 1/500 | 1/250 | 1/125 | 1/60 | 1/30 | 1/15 | 1/8 | 1/4 | 1/2 | 1 |
| 13 | 1/8000 | 1/4000 | 1/2000 | 1/1000 | 1/500 | 1/250 | 1/125 | 1/60 | 1/30 | 1/15 | 1/8 | 1/4 | 1/2 |
| 14 | 1/8000 | 1/4000 | 1/2000 | 1/1000 | 1/500 | 1/250 | 1/125 | 1/60 | 1/30 | 1/15 | 1/8 | 1/4 | |
| 15 | 1/8000 | 1/4000 | 1/2000 | 1/1000 | 1/500 | 1/250 | 1/125 | 1/60 | 1/30 | 1/15 | 1/8 | ||
| 16 | 1/8000 | 1/4000 | 1/2000 | 1/1000 | 1/500 | 1/250 | 1/125 | 1/60 | 1/30 | 1/15 | |||
| 17 | 1/8000 | 1/4000 | 1/2000 | 1/1000 | 1/500 | 1/250 | 1/125 | 1/60 | 1/30 | ||||
| 18 | 1/8000 | 1/4000 | 1/2000 | 1/1000 | 1/500 | 1/250 | 1/125 | 1/60 | |||||
| 19 | 1/8000 | 1/4000 | 1/2000 | 1/1000 | 1/500 | 1/250 | 1/125 | ||||||
| 20 | 1/8000 | 1/4000 | 1/2000 | 1/1000 | 1/500 | 1/250 | |||||||
| 21 | 1/8000 | 1/4000 | 1/2000 | 1/1000 | 1/500 | ||||||||
| EV | 1.0 | 1.4 | 2.0 | 2.8 | 4.0 | 5.6 | 8.0 | 11 | 16 | 22 | 32 | 45 | 64 |
| f-number | |||||||||||||
Tabulated exposure values
An exposure meter may not always be available, and using a meter todetermine 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
Lighting Condition EV100 Daylight Light sand or snow in full or slightly hazy sunlight (distinct shadows)a 16 Typical scene in full or slightly hazy sunlight (distinct shadows)a, b 15 Typical scene in hazy sunlight (soft shadows) 14 Typical scene, cloudy bright (no shadows) 13 Typical scene, heavy overcast 12 Areas in open shade, clear sunlight 12 Outdoor, Natural light Rainbows Clear sky background 15 Cloudy sky background 14 Sunsets and skylines Just before sunset 12–14 At sunset 12 Just after sunset 9–11 The Moon,c altitude > 40° Full 15 Gibbous 14 Quarter 13 Crescent 12 Moonlight, Moon altitude > 40° Full −3 to −2 Gibbous −4 Quarter −6 Aurora borealis and australis Bright −4 to −3 Medium −6 to −5 Outdoor, Artificial Light Neon and other bright signs 9–10 Night sports 9 Fires and burning buildings 9 Bright street scenes 8 Night street scenes and window displays 7–8 Night vehicle traffic 5 Fairs and amusement parks 7 Christmas tree lights 4–5 Floodlit buildings, monuments, and fountains 3–5 Distant views of lighted buildings 2 Indoor, Artificial Light Galleries 8–11 Sports events, stage shows, and the like 8–9 Circuses, floodlit 8 Ice shows, floodlit 9 Offices and work areas 7–8 Home interiors 5–7 Christmas tree lights 4–5
- 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 - This is approximately the value given by the sunny 16 rule.
- 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
Reciprocity (photography)
In photography reciprocity refers to the inverse relationship between the intensity and duration of light that determines the reaction of light-sensitive material. Within a normal exposure range for film stock, for example, the reciprocity law states that the film response will be determined by the...
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
,increase the exposure values (decrease the exposures) by the number of exposure steps by which that speed is greater than
ISO 100, formally
For example, ISO 400 speed is two steps greater than ISO 100:
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
.
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:
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
.
Setting EV on a camera
On most cameras, there is no direct way to transfer an EV to camera settings; however, a few cameras, such as some VoigtländerVoigtländer
Voigtländer is an optical company founded by Johann Christoph Voigtländer in Vienna in 1756 and is thus the oldest name in cameras. It produced the Petzval photographic lens in 1840, and the world's first all-metal daguerrotype camera in 1841, also bringing out plate cameras shortly afterwards...
and Braun
Carl Braun camera-werk
Carl Braun Camera-Werk of Nuremberg, Germany, or Braun, as it was more commonly called, was originally founded as an optical production house. It is best known for its 35mm film cameras, the Paxettes, and for slide projectors.-History:...
models or the Kodak Pony II shown in the photo, allowed direct setting of exposure value.
Some medium-format cameras from Rollei
Rollei
Rollei is a German manufacturer of optical goods founded in 1920 by Paul Franke and Reinhold Heidecke in Braunschweig, Lower Saxony, and maker of the Rolleiflex and Rolleicord series of cameras...
(Rolleiflex
Rolleiflex
Rolleiflex is the name of a long-running and diverse line of high-end cameras originally made by the German company Franke & Heidecke, and later Rollei-Werk. The "Rolleiflex" name is most commonly used to refer to Rollei's premier line of medium format twin lens reflex cameras...
, Rolleicord
Rolleicord
The Rolleicord was a popular medium-format twin lens reflex camera made by Franke & Heidecke between 1933 and 1976. It was a simpler, less expensive version of the high-end Rolleiflex TLR, aimed at amateur photographers who wanted a high-quality camera but could not afford the expensive Rolleiflex...
models) and Hasselblad
Hasselblad
Victor Hasselblad AB is a Swedish manufacturer of medium-format cameras and photographic equipment based in Gothenburg, Sweden.The company is best known for the medium-format cameras it has produced since World War II....
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
Shutter speed
In photography, shutter speed is a common term used to discuss exposure time, the effective length of time a camera's shutter is open....
or aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
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).
Exposure compensation in EV
Many current cameras allow for exposure compensationExposure compensation
Exposure compensation is a technique for adjusting the exposure indicated by a photographic exposure meter, in consideration of factors that may cause the indicated exposure to result in a less-than-optimal image. Factors considered may include unusual lighting distribution, variations within a...
, 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
Exposure compensation
Exposure compensation is a technique for adjusting the exposure indicated by a photographic exposure meter, in consideration of factors that may cause the indicated exposure to result in a less-than-optimal image. Factors considered may include unusual lighting distribution, variations within a...
of +1 EV
(or +1 step) means to increase exposure, by using either a longer exposure time or a smaller
-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.
Meter indication in EV
Some light meterLight meter
A light meter is a device used to measure the amount of light. In photography, a light meter is often used to determine the proper exposure for a photograph...
s (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
-numbers.Recently, articles on many web sites have used light value
Light value
In photography, light value has been used to refer to a“light level” for either incident or reflected light, often on a base-2 logarithmic scale.The term does not derive from a published standard, and has had severaldifferent meanings:...
(LV) to
denote EV at ISO 100. However, this term does not derive from a standards
body, and has had several conflicting definitions.
Relationship of EV to lighting conditions
The recommended f-number and exposure time for given lighting conditionsand ISO speed are given by the exposure equation
where
- N is the relative apertureApertureIn optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
(f-numberF-numberIn optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...
) - t is the exposure time (“shutter speedShutter speedIn photography, shutter speed is a common term used to discuss exposure time, the effective length of time a camera's shutter is open....
”) in seconds - L is the average scene luminanceLuminanceLuminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
- S is the ISO arithmetic speedFilm speedFilm speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system....
- K is the reflected-light meter calibration constant
Applied to the right-hand side of the exposure equation, exposure value is
Camera settings also can be determined from incident-light measurements,
for which the exposure equation is
where
- E is the illuminanceIlluminanceIn photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per...
- C is the incident-light meter calibration constant
In terms of exposure value, the right-hand side becomes
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.
EV and APEX
The Additive system of Photographic EXposure (APEXAPEX system
APEX stands for Additive system of Photographic EXposure, whichwas proposed in the 1960 ASA standardfor monochrome film speed, ASA PH2.5-1960,as a means of simplifying exposure computation.-Exposure equation:...
)
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
where Av (aperture value) and Tv (time value) were defined as:
and
with
- A the relative aperture (f-number)
- T the exposure time (“shutter speed”) in seconds
Av and Tv represent the numbers of stops from 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.
EV as a measure of luminance and illuminance
For a given ISO speed and meter calibration constant, there is a direct relationshipbetween 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
Canon Inc.
is a Japanese multinational corporation that specialises in the manufacture of imaging and optical products, including cameras, camcorders, photocopiers, steppers and computer printers. Its headquarters are located in Ōta, Tokyo, Japan.-Origins:...
, Nikon
Nikon
, also known as just Nikon, is a multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging. Its products include cameras, binoculars, microscopes, measurement instruments, and the steppers used in the photolithography steps of semiconductor fabrication, of which...
, and Sekonic
Mamiya
is a Japanese company that today manufactures high-end cameras and other related photographic and optical equipment. With headquarters in Tokyo, it has two manufacturing plants and a workforce of over 200 people...
).
Using , 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
Cardioid
A cardioid is a plane curve traced by a point on the perimeter of a circle that is rolling around a fixed circle of the same radius. It is therefore a type of limaçon and can also be defined as an epicycloid having a single cusp...
-responding).
Illuminance is measured with a flat sensor; a typical value for
C is 250 with illuminance in lux
Lux
The lux is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface...
. Using ,
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
Light meter
A light meter is a device used to measure the amount of light. In photography, a light meter is often used to determine the proper exposure for a photograph...
article.
- Table 3. Exposure value vs. luminance (ISO 100, K = 12.5) and illuminance (ISO 100, C = 250)
EV100 Luminance Illuminance cd/m2 fL FootlambertA foot-lambert or footlambert is a unit of luminance in U.S. customary units and some other unit systems. A foot-lambert equals 1/π candela per square foot, or 3.426 candela per square meter . The foot-lambert is named after Johann Heinrich Lambert , a Swiss-German mathematician, physicist and...
lx LuxThe lux is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface...
fc Foot-candleA foot-candle is a non-SI unit of illuminance or light intensity widely used in photography, film, television, conservation lighting, and the lighting industry...
−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
See also
- APEX systemAPEX systemAPEX stands for Additive system of Photographic EXposure, whichwas proposed in the 1960 ASA standardfor monochrome film speed, ASA PH2.5-1960,as a means of simplifying exposure computation.-Exposure equation:...
- Exposure compensationExposure compensationExposure compensation is a technique for adjusting the exposure indicated by a photographic exposure meter, in consideration of factors that may cause the indicated exposure to result in a less-than-optimal image. Factors considered may include unusual lighting distribution, variations within a...
- Exposure meter calibration
- High dynamic range imagingHigh dynamic range imagingIn image processing, computer graphics, and photography, high dynamic range imaging is a set of techniques that allows a greater dynamic range between the lightest and darkest areas of an image than current standard digital imaging techniques or photographic methods...
Further reading
- Eastman Kodak Company. Existing-Light Photography, 3rd ed. Rochester, NY: Silver Pixel Press, 1996. ISBN 0-87985-744-7
External links
- Doug Kerr’s The Additive System for Photographic Exposure (PDF)
- Fred Parker’s table of exposure values for various lighting situations