Doug Kerr
Well-known member
This is specially for Tom Dinning.
We regularly see the luminance of a scene expressed in terms of the quantity "Ev". Sadly, this is inappropriate.
APEX
The Quantity Ev (exposure value) is a creature of the APEX system (additive system of photographic exposure). This system defines a logarithmic expression of various quantities of interest in the area of photographic exposure.
It intent was to allow photographers to "solve the standard exposure equation on their fingers" by giving a form of that equation that involved only addition, not multiplication or division.
By the time the system was introduced, it had almost no value to the "working photographer", since exposure metering (and automatic exposure control) was almost universally available, and all the calculations were made for us.
But the logarithmic scheme it established was very useful in "theoretical" work.
The standard exposure equation
We can see the principal of APEX, and learn of four of its import quantities, by looking at the APEX-form of the "standard exposure equation. This equation suggest a photographic exposure (combination of aperture and shutter speed) based on the measured or assumed average luminance of the scene and the ISO speed of the film. The equation is:
Av + Tv = Bv + Sv
Here, Av (aperture value) is a binary logarithmic representation of the aperture. It is 1.0 for f/1.0, and increases by one unit for each "one stop" decrease in aperture.
Tv (time value) is a binary logarithmic representation of the exposure time. It is 1.0 for 1.0 second, and increases by one unit for each cutting in half of the time.
Bv (brightness value) is a binary logarithmic representation of the measured or assumed average scene luminance. I will not discuss the unit basis here, but Bv, and increases by one unit for each doubling of the luminance.
Sv (speed value) is a binary logarithmic representation of the assumed ISO speed of the film (the exposure index). It is 5.0 for ISO 100, and increases by one unit for each doubling of the ISO speed.
Yes, this exposure equation is naïve, but it is the same naïveté of classical reflected-light exposure meters.
The quantity Ev (exposure value)
We now define a derived unit, Ev (exposure value), which will reflect the joint effect on exposure of the aperture and exposure time:
Ev = Av + Tv
Thus we can recast the exposure equation this way:
Ev = Bv + Sv
Now, if we know (or can assume) the scene luminance, and know what exposure index we wish to use (perhaps the announced ISO sensitivity of the film), we can determine the recommended photographic exposure (Ev). We can then play off aperture vs exposure time to meet our other criteria.
In fact,at one time, some cameras had on their shutters a scale showing the Ev for any aperture and shutter speed setting. And many exposure meters also had an Ev scale (the basic "recommendation" of the exposure meter is actually in terms of that quantity, so why not show it on the "exposure calculator").
Of all the quantities defined in the APEX system (there are some I did not mention), Ev was the only one ever seen by many photographers.
Expressing scene luminance
There are many occasions in which we want to express a value of scene luminance. We may wish to say that the exposure metering system of a certain camera will work for scene luminance down to a certain point. We may wish to say that it is practice, while meeting various criteria, to properly photograph scenes with a luminance down to a certain value.
The average working photographer is not used to dealing with the various units of luminance. So it was felt attractive to use the brightness quantity of APEX as the basis for such statements. That is of course Bv.
But no photographer still alive (present company accepted) had ever heard of Bv. But some had heard of Ev (which measures something totally different).
So the crafty camera manufacturers adopted this convention: We will state a certain scene luminance by stating the Ev that the standard exposure equation would "recommend" for a scene of that luminance and an exposure index of ISO 100.
So this would make obvious sense to the working photographer. Certainly she could intuitively appreciate what scenes would lead her to use an photographic exposure of Ev 14 if she were shooting at an exposure index of ISO 100. Let's see - what kind of photographic exposure would Ev 14 be?
Well, of course not.
So the use of Ev in this warped way is of no help to the "reader". It would be much better to use the proper APEX quantity, Bv. The working photographer cannot grasp that intuitively either, but it has the advantage of being appropriate.
By the way, if we assume an exposure index of ISO 100 (as in the "convention" for the use of Ev to describe scene luminance), then:
Bv = Ev -5
So for the oft-quoted "Ev 14", the actual luminance (in APEX form) is Bv 9.
Just to give a real-life reference, for the scene luminance assumed by the "sunny 16" rule of thumb for reckoning exposure, and for the average scene reflectance assumed by other parts of the classical exposure measurement doctrine, the average scene luminance would be Bv 9.6. Under the unfortunate "Ev" convention, that would be said to be Ev 14.6.
Lovely day today here in Alamogordo. About "sunny 20" (fall has already begun to set in).
Tom, there will be a quiz.
Best regards,
Doug
We regularly see the luminance of a scene expressed in terms of the quantity "Ev". Sadly, this is inappropriate.
APEX
The Quantity Ev (exposure value) is a creature of the APEX system (additive system of photographic exposure). This system defines a logarithmic expression of various quantities of interest in the area of photographic exposure.
It intent was to allow photographers to "solve the standard exposure equation on their fingers" by giving a form of that equation that involved only addition, not multiplication or division.
By the time the system was introduced, it had almost no value to the "working photographer", since exposure metering (and automatic exposure control) was almost universally available, and all the calculations were made for us.
But the logarithmic scheme it established was very useful in "theoretical" work.
The standard exposure equation
We can see the principal of APEX, and learn of four of its import quantities, by looking at the APEX-form of the "standard exposure equation. This equation suggest a photographic exposure (combination of aperture and shutter speed) based on the measured or assumed average luminance of the scene and the ISO speed of the film. The equation is:
Av + Tv = Bv + Sv
Here, Av (aperture value) is a binary logarithmic representation of the aperture. It is 1.0 for f/1.0, and increases by one unit for each "one stop" decrease in aperture.
Tv (time value) is a binary logarithmic representation of the exposure time. It is 1.0 for 1.0 second, and increases by one unit for each cutting in half of the time.
Bv (brightness value) is a binary logarithmic representation of the measured or assumed average scene luminance. I will not discuss the unit basis here, but Bv, and increases by one unit for each doubling of the luminance.
Sv (speed value) is a binary logarithmic representation of the assumed ISO speed of the film (the exposure index). It is 5.0 for ISO 100, and increases by one unit for each doubling of the ISO speed.
Yes, this exposure equation is naïve, but it is the same naïveté of classical reflected-light exposure meters.
The quantity Ev (exposure value)
We now define a derived unit, Ev (exposure value), which will reflect the joint effect on exposure of the aperture and exposure time:
Ev = Av + Tv
Thus we can recast the exposure equation this way:
Ev = Bv + Sv
Now, if we know (or can assume) the scene luminance, and know what exposure index we wish to use (perhaps the announced ISO sensitivity of the film), we can determine the recommended photographic exposure (Ev). We can then play off aperture vs exposure time to meet our other criteria.
In fact,at one time, some cameras had on their shutters a scale showing the Ev for any aperture and shutter speed setting. And many exposure meters also had an Ev scale (the basic "recommendation" of the exposure meter is actually in terms of that quantity, so why not show it on the "exposure calculator").
Of all the quantities defined in the APEX system (there are some I did not mention), Ev was the only one ever seen by many photographers.
Expressing scene luminance
There are many occasions in which we want to express a value of scene luminance. We may wish to say that the exposure metering system of a certain camera will work for scene luminance down to a certain point. We may wish to say that it is practice, while meeting various criteria, to properly photograph scenes with a luminance down to a certain value.
The average working photographer is not used to dealing with the various units of luminance. So it was felt attractive to use the brightness quantity of APEX as the basis for such statements. That is of course Bv.
But no photographer still alive (present company accepted) had ever heard of Bv. But some had heard of Ev (which measures something totally different).
So the crafty camera manufacturers adopted this convention: We will state a certain scene luminance by stating the Ev that the standard exposure equation would "recommend" for a scene of that luminance and an exposure index of ISO 100.
So this would make obvious sense to the working photographer. Certainly she could intuitively appreciate what scenes would lead her to use an photographic exposure of Ev 14 if she were shooting at an exposure index of ISO 100. Let's see - what kind of photographic exposure would Ev 14 be?
Hint: Well, of course, that could be, for example. f/8 at 1/250 sec, or f/11 at 1/125 sec. Anybody would know that.
Well, of course not.
So the use of Ev in this warped way is of no help to the "reader". It would be much better to use the proper APEX quantity, Bv. The working photographer cannot grasp that intuitively either, but it has the advantage of being appropriate.
By the way, if we assume an exposure index of ISO 100 (as in the "convention" for the use of Ev to describe scene luminance), then:
Bv = Ev -5
So for the oft-quoted "Ev 14", the actual luminance (in APEX form) is Bv 9.
Just to give a real-life reference, for the scene luminance assumed by the "sunny 16" rule of thumb for reckoning exposure, and for the average scene reflectance assumed by other parts of the classical exposure measurement doctrine, the average scene luminance would be Bv 9.6. Under the unfortunate "Ev" convention, that would be said to be Ev 14.6.
Lovely day today here in Alamogordo. About "sunny 20" (fall has already begun to set in).
Tom, there will be a quiz.
Best regards,
Doug