Doug Kerr
Well-known member
Our discussions here about many important topics (like "exposing to the right") are often hampered by the fact that there are two quite different, but equally legitimate, meanings of the term "exposure". I thought I would talk a little about this and make some proposals as to how we can avoid confusion.
In my own technical articles, I often take the extreme measure of using two coined terms, "exposure1" and "exposure2", to avoid ambiguity. I'm not proposing that as our general solution, but I will use it here for a little while just so I can make clear what two things I am talking about. The brief definitions of these two terms are as follows:
Exposure1 - the measure of joint effect of exposure time (shutter speed) and relative aperture (f/number).
Exposure2 - the measure of the physical phenomenon to which film, or a digital sensor, responds, formally called "photometric exposure". It is the product of the illuminance on the medium and the time it persists (the exposure time). Note that illuminance is the amount of luminous flux (the "stuff of light") falling on each unit of the surface of the medium.
Now let me expand.
"Exposure1" (in its basic mathematical form) does not have a standard scientific symbol (nor even an unambiguous name - it is often called just "exposure").
But there is a standard symbol we can easily use to mention this quantity, and that is "Ev" (so long as we don't get sucked into the "other use of Ev" to describe scene luminance). Ev, numerically, is a logarithmic expression of exposure1. If we cut the shutter speed in half, or decrease the aperture (increase the f/number) by "one stop", Ev increases by one unit.
I suggest that when we are speaking of exposure1 in the narrative sense we just use "Ev" ("If we keep the same Ev, then we can . . .")
Note that if we allow into our writing the unfortunate convention is which Ev is also used to express scene luminance, it will mess this up. So don't do that. There is a perfectly good factor, in the same vein as Ev (logarithmic) for scene luminance: Bv. More about that in another note.
"Exposure2" (photometric exposure) today has the scientific symbol H. It formerly had the symbol E, as in the traditional description of the curve of negative film response, the "D log E" curve. But E has another use in photometric science (it represents illuminance) and so as these things got sorted out by the boffins, H was adopted for photometric exposure.
I suggest that when we are speaking of photometric exposure, we call it "photometric exposure" or (once we get rolling), just "H", as in: "Then, the brightest spot on the scene will receive, on the sensor, an H that is almost the largest H the sensor can distinguish".
I'll generally be following these practices in my correspondence in these forums.
In my own technical articles, I often take the extreme measure of using two coined terms, "exposure1" and "exposure2", to avoid ambiguity. I'm not proposing that as our general solution, but I will use it here for a little while just so I can make clear what two things I am talking about. The brief definitions of these two terms are as follows:
Exposure1 - the measure of joint effect of exposure time (shutter speed) and relative aperture (f/number).
Exposure2 - the measure of the physical phenomenon to which film, or a digital sensor, responds, formally called "photometric exposure". It is the product of the illuminance on the medium and the time it persists (the exposure time). Note that illuminance is the amount of luminous flux (the "stuff of light") falling on each unit of the surface of the medium.
Now let me expand.
"Exposure1" (in its basic mathematical form) does not have a standard scientific symbol (nor even an unambiguous name - it is often called just "exposure").
But there is a standard symbol we can easily use to mention this quantity, and that is "Ev" (so long as we don't get sucked into the "other use of Ev" to describe scene luminance). Ev, numerically, is a logarithmic expression of exposure1. If we cut the shutter speed in half, or decrease the aperture (increase the f/number) by "one stop", Ev increases by one unit.
I suggest that when we are speaking of exposure1 in the narrative sense we just use "Ev" ("If we keep the same Ev, then we can . . .")
Note that if we allow into our writing the unfortunate convention is which Ev is also used to express scene luminance, it will mess this up. So don't do that. There is a perfectly good factor, in the same vein as Ev (logarithmic) for scene luminance: Bv. More about that in another note.
"Exposure2" (photometric exposure) today has the scientific symbol H. It formerly had the symbol E, as in the traditional description of the curve of negative film response, the "D log E" curve. But E has another use in photometric science (it represents illuminance) and so as these things got sorted out by the boffins, H was adopted for photometric exposure.
I suggest that when we are speaking of photometric exposure, we call it "photometric exposure" or (once we get rolling), just "H", as in: "Then, the brightest spot on the scene will receive, on the sensor, an H that is almost the largest H the sensor can distinguish".
I'll generally be following these practices in my correspondence in these forums.