Exposure value


In photography, exposure value is a number that represents a combination of a camera's shutter speed and f-number, such that all combinations that yield the same exposure have the same EV. Exposure value is also used to indicate an interval on the photographic exposure scale, with a difference of 1 EV corresponding to a standard power-of-2 exposure step, commonly referred to as a stop.
The EV concept was developed by the German shutter manufacturer Friedrich Deckel in the 1950s. Its intent was to simplify choosing among equivalent camera exposure settings by replacing combinations of shutter speed and f-number with a single number.
On some lenses with leaf shutters, the process was further simplified by allowing the shutter and aperture controls to be linked such that, when one was changed, the other was automatically adjusted to maintain the same exposure. This was especially helpful to beginners with limited understanding of the effects of shutter speed and aperture and the relationship between them. But it was also useful for experienced photographers who might choose a shutter speed to stop motion or an f-number for depth of field, because it allowed for faster adjustment—without the need for mental calculations—and reduced the chance of error when making the adjustment.
The concept became known as the Light Value System in Europe; it was generally known as the Exposure Value System when the features became available on cameras in the United States.
Because of mechanical considerations, the coupling of shutter and aperture was limited to lenses with leaf shutters; however, various automatic exposure modes now work to somewhat the same effect in cameras with focal-plane shutters.
The proper EV was determined by the scene luminance and film speed; it was intended that the system also include adjustment for filters, exposure compensation, and other variables. With all of these elements included, the camera would be set by transferring the single number thus determined.
Exposure value has been indicated in various ways. The ASA and ANSI standards used the quantity symbol, with the subscript indicating the logarithmic value; this symbol continues to be used in ISO standards, but the acronym "EV" is more common elsewhere. The Exif standard uses "Ev".
Although all camera settings with the same EV nominally give the same exposure, they do not necessarily give the same picture. The f-number determines the depth of field, and the shutter speed determines the amount of motion blur, as illustrated by the two images at the right.

Formal definition

Exposure value is a base-2 logarithmic scale, defined by:
where
The second line is just applying the quotient identity of logarithms to the first line.
EV 0 corresponds to an exposure time of and an aperture of. If the EV is known, it can be used to select combinations of exposure time and f-number, as shown in.
Each increment of 1 in exposure value corresponds to a change of one "step" 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 lower ISO speeds.

Camera settings vs. luminous exposure

"Exposure value" indicates combinations of camera settings rather than the luminous exposure, which is given by
where
The illuminance is controlled by the f-number but also depends on the scene luminance. To avoid confusion, some authors, such as Sidney Ray, 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.

Relationship of camera settings to luminous exposure

The image-plane illuminance is directly proportional to the area of the aperture, and hence inversely proportional to the square of the lens f-number; thus
for constant lighting conditions, the exposure is constant as long as the ratio is constant. If, for example, the f-number is changed, an equivalent exposure time can be determined from
Performing this calculation mentally is tedious for most photographers, but the equation is easily solved with a calculator dial on an exposure meter or a similar dial on a standalone calculator. If the camera controls have detents, constant exposure can be maintained by counting the steps as one control is adjusted and counting an equivalent number of steps when adjusting the other control.

Representing camera settings: EV

The ratio could be used to represent equivalent combinations of exposure time and f-number in a single value. But for many such combinations used in general photography, the ratio gives a fractional value with a large denominator; this is notationally inconvenient as well as difficult to remember. Inverting this ratio and taking the base-2 logarithm allows defining a quantity such that
resulting in a value that progresses in a linear sequence as camera exposure is changed in power-of-2 steps. For example, beginning with and, decreasing exposure gives the simple sequence
The last two values shown frequently apply when using ISO 100 speed imaging media in outdoor photography.
This system provides its greatest benefit when using an exposure meter calibrated in EV with a camera that allows settings to be made in EV, especially with coupled shutter and aperture; the appropriate exposure is easily set on the camera, and choosing among equivalent settings is made by adjusting one control.
Current cameras do not allow direct setting of EV, and cameras with automatic exposure control generally obviate the need for it. EV can nonetheless be helpful when used to transfer recommended exposure settings from an exposure meter to an exposure calculator.

EV as an indicator of camera settings

Used as an indicator of camera settings, EV corresponds to actual combinations of shutter speed and aperture setting. When the actual EV matches that recommended by the light level and the ISO speed, these settings should result in the "correct" exposure.
File:Exposure program chart.gif|thumb|600px|none|Popular exposure chart type, showing exposure values EV as combinations of aperture and shutter speed values. The green lines are sample program lines, by which a digital camera automatically selects both the shutter speed and the aperture for given exposure value, when set to Program mode.

Relationship of EV to lighting conditions

"Correct" exposure is obtained when the f-number and exposure time match those "recommended" for given lighting conditions
and ISO speed; the relationship is given by the exposure equation prescribed by ISO 2720:1974:
where
Applied to the right-hand side of the exposure equation, exposure value is
If the common value of /ISO is used, an EV of zero for corresponds to a luminance of . At EV = 15 the luminance is .
Camera settings also can be determined from incident-light measurements, for which the exposure equation is
where
  • is the illuminance in lux or lumens per square meter; and
  • 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,
a greater EV corresponds to greater luminance or illuminance.
Illuminance is measured using a flat sensor; if the common value of is used, an EV of zero for corresponds to an illuminance of . At EV = 15 the illuminance is . For general photography, incident-light measurements are usually taken with a hemispherical sensor; the readings cannot be meaningfully related to illuminance.

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.
Exposure values in 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 tabulated exposure values is explained in detail in the ANSI exposure guide.
The exposure values in Table 2 are for ISO 100 speed. For a different ISO speed, increase the exposure values 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 imaging medium, search Table 2 for "Night sports", and add 2 to get.
For lower ISO speed, decrease the exposure values 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", and subtract 1 to get.
The equation for correcting for ISO speed can also be solved for EV100:
For example, using ISO 400 film and setting the camera for EV 11 allows shooting night sports at a light level of EV100 = 9, in agreement with the example done the other way around above.