View source for APEX system

'''APEX''' stands for '''Additive System of Photographic Exposure''', which
was proposed in the 1960 ASA standard
for monochrome film speed, ASA PH2.5-1960,
as a means of simplifying [[Physics:Exposure (photography)|exposure]] computation.

==Exposure equation==

Until the late 1960s, cameras did not have built-in exposure meters, and
many photographers did not have external exposure meters.  Consequently,
it often was necessary to calculate exposure from
lighting conditions.  The relationship of recommended photographic exposure
to a scene's average luminance is given by the camera exposure equation

:<math>\frac {A^2} {T} = \frac {B S_x} {K} \,</math>

where

* <math>A</math> is the [[F-number|f-number]] (reciprocal of the relative [[Astronomy:Aperture|aperture]])
* <math>T</math> is the exposure time ("[[Physics:Shutter speed|shutter speed]]") in seconds
* <math>B</math> is the average scene [[Physics:Luminance|luminance]] ("[[Physics:Brightness|brightness]]")
* <math>S_x</math> is the ASA arithmetic [[Physics:Film speed|film speed]]
* <math>K</math> is the reflected-light [[Earth:Light meter#Calibration constants|meter calibration constant]]

Use of the symbol <math>B</math> for luminance reflects photographic
industry practice at the time of ASA PH2.5-1960; current SI practice prefers the symbol <math>L</math>.  German sources typically used <math>k</math> for the relative aperture.  Many authors now use
<math>N</math> and <math>t</math> for relative aperture and exposure
time.

Recommendations for the value of the calibration constant <math>K</math> in
applicable ANSI and [[Organization:International Organization for Standardization|ISO]] standards have varied slightly over the
years; this topic is discussed in greater detail under
[[Earth:Light meter#Exposure meter calibration|Exposure meter calibration]]
in the [[Earth:Light meter|Light meter]] article.

=={{anchor|LVS|EVS|EV}}Exposure value==

{{main|Physics:Exposure value}}

In an attempt to simplify choosing among combinations of equivalent camera settings, the concept of ''exposure values'' (German: ''Lichtwert'') was originally developed and proposed to other manufacturers by the German shutter manufacturer Friedrich Deckel in the early 1950s.<!-- Various Deckel patents exist, f.e. BP 744962 in 1952/1956 by Hans Deckel and Friedrich Wilhelm Deckel, US patent 2911897 in 1955/1959 by Christoph Fahl, Hans Deckel and Friedrich Wilhelm Deckel. Used in Synchro-Compur, Compur-Rapid, Prontor-SVS, etc. --> Combinations of [[Physics:Shutter speed|shutter speed]] and relative aperture that resulted in the same exposure were said to have the same ''[[Physics:Exposure value|exposure value]]'' <math>E_v</math>, a base-2 [[Logarithm|logarithm]]ic scale defined by

:<math>E_v = \log_2 {\frac {A^2} {T} } = \log_2 {\frac {B S_x} {K} } \,.</math>

When applied to the left-hand side of the exposure equation, <math>E_v</math> denoted combinations of camera settings; when applied to the right-hand side, <math>E_v</math> denoted combinations of [[Physics:Luminance|luminance]] and film speed. For a given film speed, the recommended exposure value was determined solely by the luminance. Once the exposure value was determined, it could be directly set on cameras with an <math>E_v</math> scale. Adjustment of exposure was simple, because a change of 1 <math>E_v</math> corresponded to a change of 1 exposure step, i.e., either a halving or doubling of exposure.

Starting 1954, the so-called ''Exposure Value Scale'' (EVS), originally known as ''Light Value Scale'' (LVS), was adopted by Rollei, [[Company:Hasselblad|Hasselblad]], [[Company:Voigtländer|Voigtländer]], Braun, [[Company:Kodak|Kodak]], [[Company:Seikosha|Seikosha]], Aires, [[Company:Konica|Konica]], [[Company:Olympus Corporation|Olympus]], [[Company:Ricoh|Ricoh]] and others, introducing lenses with coupled shutters and apertures, such that, after setting the exposure value, adjusting either the shutter speed or aperture made a corresponding adjustment in the other to maintain a constant exposure. On some models, the coupling of shutter speed and aperture setting was optional, so that photographers could choose their preferred method of working depending on the situation. Use of the <math>E_v</math> scale on such cameras is discussed briefly by Adams (1981, 39).

Modern cameras no longer display exposure values as such, but continue to offer exposure modes, which support users in employing the concept of counter-adjusting shutter speed and aperture at a fixed point of exposure. This can be found in features such as Manual Shift<!-- introduced with Minolta 9xi, 1992 --> on some Minolta, [[Company:Konica Minolta|Konica Minolta]] and Sony Alpha or Hyper Manual<!-- introduced with Pentax Z1, 1991 --> on some Pentax (D)SLRs since 1991, where the photographer can change one of the parameters, and the camera will adjust the other accordingly for as long as the Auto-Exposure Lock (AEL) function is activated. In a wider sense, functions like {{Interlanguage link multi|Program Shift|de|3=Programm-Shift}}<!-- introduced with Minolta 7000 AF/9000 AF, 1985 -->, Pa / Ps Creative Program Control<!-- introduced with Minolta 7xi, 1991 --> (by Minolta, Konica Minolta and Sony) or Hyper Program<!-- introduced with Pentax Z1, 1991 --> (by Pentax) belong to this group of features as well.

=={{anchor|Av|Tv|Sv|Bv}}The additive (logarithmic) system==<!-- This section is linked from [[Physics:Light value|Light value]] -->

Although some photographers (Adams 1981, 66)<ref>[[Biography:Ansel Adams|Ansel Adams]] described the exposure equation in a slightly different form:
{{quote|To use the Exposure Formula, take the film speed number (on the ASA scale) and determine its approximate square root. This number is remembered as the ''key stop'' for that film. ... At the key stop, the correct shutter speed is the reciprocal of the luminance expressed in c/ft<sup>2</sup>.}}

Using these values for <math>A</math> and <math>T</math> gives
:<math>\frac {A^2} {T} = \frac { \left ( \sqrt S \right ) ^2 } {1/L} =  LS \,.</math>
Ostensibly, the calibration constant <math>K</math> is missing, but with luminance in cd/ft<sup style="font-size: 70%">2</sup>, the value was unity (ASA Z38.2.6-1948 specified a range of 1–1.35; ASA PH2.12-1961 specified 1.06 ± 0.16. The current recommendation in ISO 2720:1974 would be 0.98–1.24 with luminance in cd/ft<sup style="font-size: 70%">2</sup>).</ref> routinely determined camera settings using the exposure equation, it generally was assumed that doing so would prove too daunting for the casual photographer.  The 1942 ASA exposure guide, ASA Z38.2.2-1942, featured a dial calculator,
and revisions in 1949 and 1955 used a similar approach.

An alternative simplification also was possible: ASA PH2.5-1960 proposed extending the concept of exposure value to all exposure parameters. Taking base-2 logarithms of both sides of the exposure equation and separating numerators and denominators reduces exposure calculation to a matter of addition:

:<math>E_v = A_v + T_v = B_v + S_v \,,</math>

where

* <math>A_v</math> is the ''aperture value'': <math>A_v = </math> <math>\log_2</math> <math>A^2</math> 
* <math>T_v</math> is the ''time value'': <math>T_v = \log_2 </math> <math>(1/T)</math>
* <math>E_v</math> is the ''exposure value'': <math>E_v = A_v + T_v</math>.
* <math>S_v</math> is the ''speed value'' (aka ''sensitivity value''): <math>S_v = \log_2</math> <math>(N S_x)</math>
* <math>B_v</math> is the ''luminance value'' (aka ''brightness value''): <math>B_v = \log_2</math> <math>(B / N K)</math>
* <math>N</math> is a constant that establishes the relationship between the ASA arithmetic film speed <math>S_x</math> and the ASA speed value <math>S_v</math>. The value of <math>N</math> is approximately 0.30 (precisely, <math>2^{-7/4}</math>).<ref>
The origin of the value of <math>2^{-7/4}</math> for <math>N</math> is arcane,
apparently so much so that ASA PH2.12-1961
included an explanation of what ASA PH2.5-1960
had intended.
</ref>

* <math>K</math> is the reflected-light meter calibration constant

ASA standards covered incident-light meters as well as reflected-light meters; the incident-light exposure equation is

:<math> \frac {A^2} {T} = \frac {I S_x} {C} \,,</math>

where

* <math>I</math> is the scene [[Physics:Illuminance|illuminance]]
* <math>C</math> is the incident-light meter calibration constant

The use of <math>I</math> for illuminance reflects photographic industry practice at the time of the 1961 ASA standard for exposure meters, ASA PH2.12-1961; current SI practice prefers the symbol <math>E</math>.

ASA PH2.12-1961 included incident-light metering in the APEX concept:

:<math>E_v = A_v + T_v = I_v + S_v \,,</math>

where

* <math>I_v</math> is the incident-light value: <math>I_v = \log_2</math> <math>(I / N C)</math>

(German sources typically use <math>LW</math> (for ''Lichtwert'' or ''Belichtungswert'' — but not to be confused with the English term [[Physics:Light value|light value]]) instead of the exposure value's symbol <math>E_v</math>. Consequently, the aperture value <math>A_v</math> is referred to as ''Blendenleitwert'' <math>LWk</math>, and the time value <math>T_v</math> as ''Zeitleitwert'' <math>LWt</math>. The film speed value <math>S_v</math> is named ''Empfindlichkeitsleitwert'', and the brightness value <math>B_v</math> is known as ''Objekthelligkeit''.)

==APEX in practice==

APEX made exposure computation a relatively simple matter; the foreword of ASA PH2.5-1960 recommended that exposure meters, exposure calculators, and exposure tables be modified to incorporate the logarithmic values that APEX required.  In many instances, this was done: the 1973 and 1986 ANSI exposure guides, ANSI PH2.7-1973 and ANSI PH2.7-1986, eliminated exposure calculator dials in favor of tabulated APEX values.  However, the logarithmic markings for aperture and shutter speed required to set the computed exposure were never incorporated in consumer cameras.  Accordingly, no reference to APEX was made in ANSI PH3.49-1971 (though it was included in the Appendix). The incorporation of exposure meters in many cameras in the late 1960s eliminated the need to compute exposure, so APEX saw little actual use.

With the passage of time, formatting of APEX quantities has varied considerably; although the <math>v</math> originally was subscript, it sometimes was given simply as lower case, and sometimes as uppercase. Treating these quantities as acronyms rather than quantity symbols probably is reasonable because several of the quantity symbols (<math>E</math>, <math>B</math>, and <math>I</math> for exposure,
luminance, and illuminance) used at the time APEX was proposed are in conflict with current preferred SI practice.

A few artifacts of APEX remain.  Canon, [[Company:Pentax|Pentax]] and [[Company:Leica Camera|Leica]] cameras use 'Av' and 'Tv' to indicate relative aperture and shutter speed as well as to symbolize aperture priority and shutter priority modes. Some Pentax DSLRs even provide a 'TAv' [[Engineering:Digital camera modes|exposure mode]] to automatically set the [[Physics:Film speed|ISO speed]] depending on the desired aperture and shutter settings, and 'Sv' (for sensitivity priority) to pre-set the ISO speed and let the camera choose the other parameters. Some meters, such as Pentax spot meters, directly indicate the exposure value for ISO 100 film speed. For a given film speed, exposure value is directly related to luminance, although the relationship depends on the reflected-light meter calibration constant <math>K</math>.  Most photographic equipment manufacturers specify metering sensitivities in EV at ISO 100 speed (the uppercase 'V' is almost universal).

It is common to express exposure increments in EV, as when adjusting exposure relative to what a light meter indicates (Ray 2000, 316). For example, an [[Exposure compensation|exposure compensation]] of +1&nbsp;EV (or +1&nbsp;step) means to increase exposure, by using either a longer exposure time or a smaller <math>f</math>-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&nbsp;EV results in a smaller EV; conversely, an exposure compensation of −1&nbsp;EV results in a greater EV.

==Use of APEX values in Exif==

APEX has seen a partial resurrection in the [[Exif]] standard, which calls for storing exposure data using APEX values.  There are some minor differences from the original APEX in both terminology and values.  The
implied value (1/3.125) for the speed scaling constant <math>N</math> given in the Exif 2.2 specification ("Exif 2.2"; JEITA 2002) differs slightly from the APEX value of <math>2^{-7/4}</math> (0.2973); with the Exif value, an ISO arithmetic film speed of 100 corresponds exactly to a speed value<ref>Exif 2.2 refers to <math>S_\mathrm {v}</math> as "film sensitivity."
</ref> <math>S_\mathrm {v}</math> of 5.

The relationship between <math>B_{\mathrm {v}}</math> and luminance depends on both the speed scaling constant <math>N</math> and the reflected-light meter calibration constant <math>K</math>:

:<math>B_\mathrm {v} = \log_2 \frac {B} {NK} \,.</math>

Because Exif 2.2 records ISO arithmetic speed rather than film sensitivity, the value of <math>N</math> affects the recorded value of <math>B_{\mathrm {v}}</math> but not the recorded film speed.

Exif 2.2 does not recommend a range of values for <math>K</math>, presumably leaving the choice to the equipment manufacturer. The example data in Annex C of Exif 2.2 give 1 footlambert for <math>B_{\mathrm {v}}</math> = 0.  This is in agreement with the APEX value for <math>B</math>, but would imply <math>K = 1/N</math>, or 3.125 with <math>B</math> in footlamberts.  With <math>B</math> in cd/m<sup style="font-size: 70%">2</sup>, this becomes 10.7, which is slightly less than the value of 12.5 recommended by ISO 2720:1974 and currently used by many manufacturers.  The difference possibly arises from rounding <math>B</math> in the example table; it also is possible that the example data simply were copied from an old ASA or ANSI standard.

==Notes==

<references/>

==References==

* <cite id="CITEREFAdams1981">Adams, Ansel.  1981. ''The Negative.'' Boston: New York Graphic Society.</cite> {{ISBN|0-8212-1131-5}}
* <cite id="CITEREFANSIPH2.7-1973">ANSI PH2.7-1973. ''American National Standard Photographic Exposure Guide''. New York: American National Standards Institute.</cite> Superseded by ANSI PH2.7-1986.
* <cite id="CITEREFANSIPH2.7-1986">ANSI PH2.7-1986. ''American National Standard for Photography &mdash; Photographic Exposure Guide''. New York: American National Standards Institute.</cite>
* <cite id="CITEREFANSIPH3.49-1971">ANSI PH3.49-1971. ''American National Standard for general-purpose photographic exposure meters (photoelectric type)''. New York: American National Standards Institute.</cite> After several revisions, this standard was withdrawn in favor of ISO 2720:1974.
* <cite id="CITEREFASAPH2.5-1960">ASA PH2.5-1960.  ''American Standard Method for Determining Speed of photographic Negative Materials (Monochrome, Continuous Tone)''.  New York: United States of America Standards Institute.</cite>
* <cite id="CITEREFASAPH2.12-1961">ASA PH2.12-1961.  ''American Standard, General-Purpose Photographic Exposure Meters (photoelectric type)''.  New York: American Standards Association.</cite>  Superseded by ANSI PH3.49-1971.
* <cite id="CITEREFASAZ38.2.2-1942">ASA Z38.2.2-1942.  ''American Emergency Standard Photographic Exposure Computer''.  New York: American Standards Association.</cite>
* <cite id="CITEREFASAZ38.2.6-1948">ASA Z38.2.6-1948. ''American Standard for General-Purpose Photographic Exposure Meters (Photoelectric Type)''. New York: American Standards Association.</cite>  Superseded by ASA PH2.12-1957.
* <cite id="CITEREFISO2720:1974"> [http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=7690 ISO 2720:1974]. ''General Purpose Photographic Exposure Meters (Photoelectric Type)&mdash;Guide to Product Specification''. International Organization for Standardization.</cite>
* <cite id="CITEREFJEITA2002">Japan Electronics and Information Technology Industries Association. 2002. [http://www.exif.org/Exif2-2.PDF JEITA CP-3451, Exchangeable image file format for digital still cameras: Exif Version 2.2] {{Webarchive|url=https://web.archive.org/web/20171224163311/http://exif.org/Exif2-2.PDF |date=2017-12-24 }} ([[PDF]]). Japan Electronics and Information Technology Industries Association.</cite>
* JEITA. ''See'' Japan Electronics and Information Technology Industries Association.
* <cite id="CITEREFRay2000">Ray, Sidney F. 2000. Camera Exposure Determination. In ''The Manual of Photography: Photographic and Digital Imaging'', 9th ed. Ed. Ralph E. Jacobson, Sidney F. Ray, Geoffrey G. Atteridge, and Norman R. Axford.  Oxford: Focal Press.</cite> {{ISBN|0-240-51574-9}}

==External links==
* [http://dougkerr.net/Pumpkin/#APEX Doug Kerr's in-depth description of APEX]

[[Category:Photographic techniques]]

{{Sourceattribution|APEX system}}