Doug Kerr
Well-known member
APEX (the Additive System of Photographic Exposure) is a system of logarithmic expression of various factors involved in the area of photographic exposure. Its primary original objective was to make it possible for calculations using the "standard exposure equation" to be made by hand using only addition. The quantities defined all have "value" in their names, intended as an arbitrary cue that the logarithmic form is meant.
The original purpose has nearly disappeared, given the widespread availability of exposure meters and automatic exposure systems.
But one APEX "value", exposure value (Ev), remains in wide use. Unfortunately, it is used both in its proper meaning and with another totally different one.
Ev quantifies the joint effect of an exposure time (shutter speed) and a relative aperture (f/number), in logarithmic fashion. If we cut the shutter speed in half, or decrease the aperture (increase the f/number) by "one stop", Ev increases by one unit.
In effect, when we let an exposure meter regard a scene, and set into it the ISO speed of the film or digital system we are using, it issues an exposure recommendation for us in terms of Ev (although it might not be marked that way). That is, it presents us with a platter of combinations of shutter speed and f/number, any one of which will have the equivalent effect on exposure (hopefully to a happy end), all of which have the same Ev.
Unhappily, a convention was adopted in which the quantity Ev is also used to quantify scene luminance. The way that works is this: when an Ev is mentioned (in this sense), it means that scene luminance which, if regarded by an exposure meter set to ISO 100, would cause the meter to issue an exposure recommendation of the mentioned Ev.
Of course, the use of the same symbol for two different quantities inevitably leads to confusion, especially in a context where we are speaking both of exposure (the quantity expressed by Ev) and scene luminance, as we often are.
One author has sought to overcome this by introducing the term "light value" (LV) to designate the logarithmic measure of scene luminance sometimes (improperly) called Ev. But there was in fact no need. The APEX system provides a perfectly good "value" to express scene luminance, "brightness value" (Bv). And historically LV had another meaning (a precursor of the "real" Ev), and thus his solution gives rise to the possibility of a new form of confusion.
The scale of Bv is not the same as "the other Ev". The two scales are related this way:
Bv = Ev' -5
where I use the symbol Ev' to mean "the other Ev".
Now. most times when we use "Ev" in the second sense it is not even with a numerical value, as in: "If this second scene has a higher Ev, then we will need to . . .". So there isn't even the slightest inconvenience in our using a correct, unambiguous term in such cases, as: "If this second scene has a higher Bv, then we will need to . . ."
And if we want to mention actual quantitative scene luminance, and we for example measure it with our exposure meter, and if it doesn't give an answer in terms of Bv (actually, many do, but that fact is obscured), we can easily take the Ev "output" when the meter is set to ISO 100 and convert it to Bv by simply subtracting 5. (The markings on the pointer scale of my Miranda Cadius meter, for the "green" range, are in fact in terms of Bv, with a 10-unit subtractor when in the "red" range. But nothing on the meter says that.)
We often find Ev used in the second sense in such places as camera specifications. For example, the manufacturer may say, "The automatic exposure system works reliably down to Ev 2". Now what do they mean? Do they mean:
--that it will work reliably down to the scene luminance that, for the ISO speed to which the camera is set, would call for an exposure (in the proper sense of Ev) of Ev 2
or
--that (regardless of the ISO speed to which the camera is set), the system will work reliably for a scene whose luminance is not lower than Bv -3
Both would be possible specifications, depending on the source of the limitation. And you can't tell which it is from the way the specification is expressed (in terms of "Ev").
I note that in its latest standard for expressing digital camera specifications, CIPA (the technical association of the Japanese camera industry) calls for scene luminance to be expressed (when it plays a role in a specification) in terms of Bv.
Those who are interested in the APEX system may wish to read my tutorial article, "APEX - The Additive System of Photographic Exposure", available here:
http://doug.kerr.home.att.net/pumpkin/index.htm#APEX
The original purpose has nearly disappeared, given the widespread availability of exposure meters and automatic exposure systems.
But one APEX "value", exposure value (Ev), remains in wide use. Unfortunately, it is used both in its proper meaning and with another totally different one.
Ev quantifies the joint effect of an exposure time (shutter speed) and a relative aperture (f/number), in logarithmic fashion. If we cut the shutter speed in half, or decrease the aperture (increase the f/number) by "one stop", Ev increases by one unit.
In effect, when we let an exposure meter regard a scene, and set into it the ISO speed of the film or digital system we are using, it issues an exposure recommendation for us in terms of Ev (although it might not be marked that way). That is, it presents us with a platter of combinations of shutter speed and f/number, any one of which will have the equivalent effect on exposure (hopefully to a happy end), all of which have the same Ev.
Unhappily, a convention was adopted in which the quantity Ev is also used to quantify scene luminance. The way that works is this: when an Ev is mentioned (in this sense), it means that scene luminance which, if regarded by an exposure meter set to ISO 100, would cause the meter to issue an exposure recommendation of the mentioned Ev.
Of course, the use of the same symbol for two different quantities inevitably leads to confusion, especially in a context where we are speaking both of exposure (the quantity expressed by Ev) and scene luminance, as we often are.
One author has sought to overcome this by introducing the term "light value" (LV) to designate the logarithmic measure of scene luminance sometimes (improperly) called Ev. But there was in fact no need. The APEX system provides a perfectly good "value" to express scene luminance, "brightness value" (Bv). And historically LV had another meaning (a precursor of the "real" Ev), and thus his solution gives rise to the possibility of a new form of confusion.
The scale of Bv is not the same as "the other Ev". The two scales are related this way:
Bv = Ev' -5
where I use the symbol Ev' to mean "the other Ev".
Now. most times when we use "Ev" in the second sense it is not even with a numerical value, as in: "If this second scene has a higher Ev, then we will need to . . .". So there isn't even the slightest inconvenience in our using a correct, unambiguous term in such cases, as: "If this second scene has a higher Bv, then we will need to . . ."
And if we want to mention actual quantitative scene luminance, and we for example measure it with our exposure meter, and if it doesn't give an answer in terms of Bv (actually, many do, but that fact is obscured), we can easily take the Ev "output" when the meter is set to ISO 100 and convert it to Bv by simply subtracting 5. (The markings on the pointer scale of my Miranda Cadius meter, for the "green" range, are in fact in terms of Bv, with a 10-unit subtractor when in the "red" range. But nothing on the meter says that.)
We often find Ev used in the second sense in such places as camera specifications. For example, the manufacturer may say, "The automatic exposure system works reliably down to Ev 2". Now what do they mean? Do they mean:
--that it will work reliably down to the scene luminance that, for the ISO speed to which the camera is set, would call for an exposure (in the proper sense of Ev) of Ev 2
or
--that (regardless of the ISO speed to which the camera is set), the system will work reliably for a scene whose luminance is not lower than Bv -3
Both would be possible specifications, depending on the source of the limitation. And you can't tell which it is from the way the specification is expressed (in terms of "Ev").
I note that in its latest standard for expressing digital camera specifications, CIPA (the technical association of the Japanese camera industry) calls for scene luminance to be expressed (when it plays a role in a specification) in terms of Bv.
Those who are interested in the APEX system may wish to read my tutorial article, "APEX - The Additive System of Photographic Exposure", available here:
http://doug.kerr.home.att.net/pumpkin/index.htm#APEX
