Exposure value
[1] The EV concept was developed by the German shutter manufacturer Friedrich Deckel in the 1950s (Gebele 1958; Ray 2000, 318).
Its intent was to simplify choosing among equivalent camera exposure settings by replacing combinations of shutter speed and f-number (e.g., 1/125 s at f/16) with a single number (e.g., 15).
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 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.
The f-number (relative aperture) determines the depth of field, and the shutter speed (exposure time) determines the amount of motion blur, as illustrated by the two images at the right (and at long exposure times, as a second-order effect, the light-sensitive medium may exhibit reciprocity failure, which is a change of light sensitivity dependent on the irradiance at the film).
Greater exposure values are appropriate for photography in more brightly lit situations, or for lower ISO speeds.
The ratio t/N2 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 Ev 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 1 s and f/1, decreasing exposure gives the simple sequence The last two values shown frequently apply when using ISO 100 speed imaging media in outdoor photography.
When the actual EV matches that recommended by the light level and the ISO speed, these settings should result in the "correct" exposure.
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 general photography, incident-light measurements are usually taken with a hemispherical sensor; the readings cannot be meaningfully related to illuminance.
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.
Some medium-format cameras from Rollei (Rolleiflex, Rolleicord models) and Hasselblad allowed EV to be set on the lenses.
On some lenses the locking was optional, so that the photographer could choose the preferred method of working depending on the situation.
Many current cameras allow for exposure compensation, and usually state it in terms of EV (Ray 2000, 316).
Recently, articles on many web sites have used light value (LV) to denote EV at ISO 100.
Use of APEX required logarithmic markings on aperture and shutter controls, however, and these never were incorporated in consumer cameras.
For a given ISO speed and meter calibration constant, there is a direct relationship between exposure value and luminance (or illuminance).
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.
Values for the reflected-light calibration constant K vary slightly among manufacturers; a common choice is 12.5 (Canon, Nikon, and Sekonic[8]).
As with luminance, common practice among photographic equipment manufacturers is to express illuminance in EV for ISO 100 speed when specifying metering range.
