Doug Kerr
Well-known member
About exposure index
In various of my writings involving exposure metering with "free-standing" exposure meters, I refer to exposure index.
I use it with this meaning:
Why do I not just say:
Two reasons:
• We may not know the actual ISO speed. For example, when we set the "ISO" setting on our Canon EOS camera to ISO 400, that does not mean that the ISO speed is asserted to be, or even probably is, ISO 400. It turns out that in reality the ISO speed is probably about ISO 566, and that another ISO measure of sensitivity, the ISO SOS, is probably about ISO SOS 400. But no assertion in that regard is made by Canon.
• We may intentionally set the exposure index of the meter to a value different from what we believe is the ISO speed of the film or digital sensor.
That might be for one of two reasons:
We most often find the term "exposure index" used only when we have done the latter of these. But I think it is important to recognize its wider significance.
The term actually goes back to the early days of photoelectric exposure meters. At the time there were no industry norms for assessing and rating the sensitivity of photographic film.
The meters had a dial, often labeled "exposure index", for adjusting the metering equation to take film sensitivity into account. The meter manufacturer (Weston, for example) provided a table, generally based on pragmatic testing in their own laboratory, giving exposure index values for various kinds of film available at the time. The values were intended to produce a good exposure result in a lot of cases.
As today, we might not set the dial to the published number for the same reasons cited above. Thus, even at that time, a good definition of "exposure index" was "what we tell the meter is the sensitivity of the film".
The phrase "exposure index" now appears in a formal ISO "measure" of digital camera sensitivity, the ISO Recommended Exposure Index (ISO REI). It is not intended to be determined under any specific measurement doctrine. Rather, it can be thought of as the value the manufacturer suggests be used as the exposure index setting of an exposure meter (or integrated automatic exposure control system) to produce a good exposure result in a lot of cases.
Edward Weston must be smiling in his grave.
Best regards,
Doug
In various of my writings involving exposure metering with "free-standing" exposure meters, I refer to exposure index.
I use it with this meaning:
What we tell the exposure meter is the ISO speed of the film or digital sensor to be used for the shot.
Why do I not just say:
The ISO speed of the film or digital sensor to be used for the shot.
Two reasons:
• We may not know the actual ISO speed. For example, when we set the "ISO" setting on our Canon EOS camera to ISO 400, that does not mean that the ISO speed is asserted to be, or even probably is, ISO 400. It turns out that in reality the ISO speed is probably about ISO 566, and that another ISO measure of sensitivity, the ISO SOS, is probably about ISO SOS 400. But no assertion in that regard is made by Canon.
• We may intentionally set the exposure index of the meter to a value different from what we believe is the ISO speed of the film or digital sensor.
That might be for one of two reasons:
• We may wish to compensate for the fact that in certain scenes (strongly backlit subjects, etc) the conclusion of the exposure meter as to what exposure should be used is not expected to be the best or our purposes ("exposure bias").
• We may wish to shoot with a lesser photographic exposure than the meter would normally recommend (perhaps to get the shutter speed into an attractive region), expecting to compensate for that in development (chemical or digital).
• We may wish to shoot with a lesser photographic exposure than the meter would normally recommend (perhaps to get the shutter speed into an attractive region), expecting to compensate for that in development (chemical or digital).
We most often find the term "exposure index" used only when we have done the latter of these. But I think it is important to recognize its wider significance.
The term actually goes back to the early days of photoelectric exposure meters. At the time there were no industry norms for assessing and rating the sensitivity of photographic film.
The meters had a dial, often labeled "exposure index", for adjusting the metering equation to take film sensitivity into account. The meter manufacturer (Weston, for example) provided a table, generally based on pragmatic testing in their own laboratory, giving exposure index values for various kinds of film available at the time. The values were intended to produce a good exposure result in a lot of cases.
As today, we might not set the dial to the published number for the same reasons cited above. Thus, even at that time, a good definition of "exposure index" was "what we tell the meter is the sensitivity of the film".
The phrase "exposure index" now appears in a formal ISO "measure" of digital camera sensitivity, the ISO Recommended Exposure Index (ISO REI). It is not intended to be determined under any specific measurement doctrine. Rather, it can be thought of as the value the manufacturer suggests be used as the exposure index setting of an exposure meter (or integrated automatic exposure control system) to produce a good exposure result in a lot of cases.
Edward Weston must be smiling in his grave.
Best regards,
Doug
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