APEX stands for Additive System of Photographic Exposure.

This system uses log-base-2 units, known as stops or Ev units, to quantify aperture, shutter speed, film speed, brightness (luminance), lighting (illuminance), and compensations to exposure. In optics, an aperture is something which restricts the diameter of the light path through one plane in an optical system. ... The shutter speed dial of a Fujica STX-1. ... Film speed is the measure of a photographic film stocks sensitivity to light. ... Brightness is an attribute of visual perception in which a source appears to emit a given amount of light. ... Luminance (also called luminosity) is a photometric measure of the density of luminous intensity in a given direction. ... Lighting refers to either artificial light sources such as lamps or to natural illumination of interiors from daylight. ... Illuminance is the total luminous flux incident per unit area. ... A photograph with an exposure time of 25 seconds In photography, exposure is the total amount of light allowed to fall on the sensor (photographic film or CCD) during the process of taking a photograph. ...

• Av is the Aperture value, to encode f-number: f/1 => Av=0, f/1.4 => Av=1, f/2 => Av=2, f/2.8 => Av=3, etc.
• Tv is the Time value, to encode shutter speed: 1 sec => Tv=0, 1/2 sec => Tv = 1, etc.
• Ev is the Exposure value, Ev = Av + Tv, to encode the net exposure.
• Sv is the Speed value, to encode ISO film speed: ISO 3.125 => Sv=0, ISO 100 => Sv=5, ISO 200 => Sv=6, etc.
• Bv is the Brightness value, to encode metered scene luminance: 1 footlambert or 3.4 candela per square meter => Bv=0, 2 foot-lambert => Bv=1, etc. [See note 1]
• Iv is the Incident light value, to encode illuminance incident on the scene: 6.25 foot-candles or 67 Lux => Iv=0, 12.5 foot-candle => Iv=1, 25 foot-candle => Iv=2, etc.

The basic exposure equation for reflected light metering is A 35mm lens set to f/11, as indicated by the white dot above the f-stop scale on the aperture ring In photography the f-number (focal ratio) expresses the diameter of the diaphragm aperture in terms of the effective focal length of the lens. ... The time value of an option is determined by the chance that the option becomes (more) profitable and depends on the time until the expiration date and the volatility of the option. ... The shutter speed dial of a Fujica STX-1. ... Fast shutter speed, short exposure Slow Shutter speed, long exposure In photography, exposure value (EV) is a value given to all combinations of camera shutter speed and aperture that gives the same exposure. ... A photograph with an exposure time of 25 seconds In photography, exposure is the total amount of light allowed to fall on the sensor (photographic film or CCD) during the process of taking a photograph. ... Film speed is the measure of a photographic film stocks sensitivity to light. ... Luminance (also called luminosity) is a photometric measure of the density of luminous intensity in a given direction. ... The unit of measurement of brightness (in the USA). ... The candela (symbol: cd) is the SI base unit of luminous intensity (that is, power emitted by a light source in a particular direction, with wavelengths weighted by the luminosity function, a standardized model of the sensitivity of the human eye). ... Illuminance is the total luminous flux incident per unit area. ... A foot-candle (sometimes designated footcandle; abbreviated fc, lm/ft², or sometimes ft-c) is a non-SI unit of illuminance or light intensity. ... The lux (symbol: lx) is the SI derived unit of illuminance or illumination. ...

Ev = Av + Tv = Bv + Sv

and for incident light metering looks the same, assuming meter calibration based on a typical scene of 16% reflectance (1 foot-lambert luminance per 6.25 foot-candle illuminance):

Ev = Av + Tv = Iv + Sv

The small v is sometimes written as a subscript, and sometimes as a capital V.

• Lv, Light value, is an unoffical adaptation of Bv, which has been used several ways:
  • to offset Bv to an ISO-100 Ev; that is, Lv = Bv+5; this is a different and incompatible scaling of scene luminance, with values higher than the other Lv interpretation by about 5.
  • Light value has also been used as a synonymn for brightness value; in some cases using the symbol Lv^[1] and in other cases the symbol Bv^[2].

Exposure compensation relative to what the light meter suggests is often specified as Ev units, or stops of adjustment to Ev. A compensation of +1 Ev, or an Ev compensation of +1 stop, for example, means to expose with a longer shutter speed or lower f-number; that is, to subtract 1 from the Ev suggested by the meter, since higher Ev numbers mean less exposure. The factual accuracy of this article is disputed. ...

Note 1 (First, by way of context, note that the APEX system was originally intoduced for use in connection with photographic exposure meters.) Formally, the scales of Bv and Lv are not fixed at the exact relationships stated above. The scale varies with the value of the pertinent exposure metering calibration constant, K (the reflected light metering constant) in the case of Bv, and C (the incident light metering constant) in the case of Lv. The reason is so that the APEX-form exposure metering equations (given above) will not show the term for the metering constant (even though it is really in there, buried in the scale defintion for Bv or Lv).

References

1. ^ Leslie Stroebel, John Compton, Ira Current, Richard D. Zakia, Basic Photographic Materials and Processes, Focal Press, 2000
2. ^ Richard D. Zakia, Focal Encyclopedia of Photography, Butterworth-Heinemann, 1993

External Links

• Doug Kerr's in-depth description of APEX