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.
Until the late 1960s, cameras did not have built-in exposure meters, andmany photographers did not have external exposure meters. Consequently,it often was necessary to calculate exposure fromlighting conditions. The relationship of recommended photographic exposureto a scene's average luminance is given by the camera exposure equation
A2 | |
T |
=
BSx | |
K |
where
A
T
B
Sx
K
Use of the symbol
B
L
k
N
t
Recommendations for the value of the calibration constant
K
See main article: 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. Combinations of shutter speed and relative aperture that resulted in the same exposure were said to have the same exposure value
Ev
Ev=log2{
A2 | |
T |
}=log2{
BSx | |
K |
}.
When applied to the left-hand side of the exposure equation,
Ev
Ev
Ev
Ev
Starting 1954, the so-called Exposure Value Scale (EVS), originally known as Light Value Scale (LVS), was adopted by Rollei, Hasselblad, Voigtländer, Braun, Kodak, Seikosha, Aires, Konica, Olympus, 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
Ev
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 on some Minolta, Konica Minolta and Sony Alpha or Hyper Manual 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, Pa / Ps Creative Program Control (by Minolta, Konica Minolta and Sony) or Hyper Program (by Pentax) belong to this group of features as well.
Although some photographers (Adams 1981, 66)[1] 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:
Ev=Av+Tv=Bv+Sv,
where
Av
Av=
log2
A2
Tv
Tv=log2
(1/T)
Ev
Ev=Av+Tv
Sv
Sv=log2
(NSx)
Bv
Bv=log2
(B/NK)
N
Sx
Sv
N
2-7/4
K
ASA standards covered incident-light meters as well as reflected-light meters; the incident-light exposure equation is
A2 | |
T |
=
ISx | |
C |
,
where
I
C
The use of
I
E
ASA PH2.12-1961 included incident-light metering in the APEX concept:
Ev=Av+Tv=Iv+Sv,
where
Iv
Iv=log2
(I/NC)
(German sources typically use
LW
Ev
Av
LWk
Tv
LWt
Sv
Bv
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
v
E
B
I
A few artifacts of APEX remain. Canon, Pentax and 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' exposure mode to automatically set the 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
K
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 of +1 EV (or +1 step) means to increase exposure, by using either a longer exposure time or a smaller
f
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. Theimplied value (1/3.125) for the speed scaling constant
N
2-7/4
Sv
The relationship between
Bv
N
K
Bv=log2
B | |
NK |
.
Because Exif 2.2 records ISO arithmetic speed rather than film sensitivity, the value of
N
Bv
Exif 2.2 does not recommend a range of values for
K
Bv
B
K=1/N
B
B
B
2-7/4
N
Sv