Doug Kerr
Well-known member
In discussions of exposure metering and such we often see mention of the value "18%". But just what that represents, and under what conditions, is often unclear - and some of the statements we see are just wrong.
The situation is complicated for the researcher by the fact that this number emerges from the interaction of several ISO standards, none of them prescribing or assuming it directly, each seemingly assuming that is has been defined or assumed by another standard, often indirectly.
If we follow this trail of shadows, the following seems to be the case:
• The trail of reflective light photographic exposure metering starts with the assumption that the "representative" scene:
•• Has an average reflectance of 18% (aha!).
•• Has a maximum reflectance of 100%
But, as "seen" by the exposure meter, and later the camera, this means:
• The average luminance of the scene is 18% of its maximum luminance.
If we follow the trail through the ISO standard for the sensitivity of digital cameras and the ISO standard for photographic exposure meters, and choose an appropriate value for the reflected light metering calibration constant, K, we find that for a metered exposure, the average photometric exposure on the camera sensor will always be (regardless of the scene particulars) about 12.7% of the saturation exposure (the highest exposure at which the imaging system cam properly determine differences in scene luminance—that is, properly record detail).
Now for our "representative" scene, with an average luminance of 18% of its maximum luminance, that means on the sensor the greatest phtometric expose will be about 70.6% of the saturation exposure - that is, almost exactly 1/2 stop short of saturation. That 1/2 stop "clearance" is viewed as "headroom" to accommodate scenes that do not fit the "representative" model.
Note that this only works out this way if we choose the "appropriate" value of K for the exposure meter. There is no "correct" value of K - that can be chosen by the exposure meter manufacture from a range given in the ISO standard to "tune" the meter to what they feel is the best for users.
Best regards,
Doug
The situation is complicated for the researcher by the fact that this number emerges from the interaction of several ISO standards, none of them prescribing or assuming it directly, each seemingly assuming that is has been defined or assumed by another standard, often indirectly.
If we follow this trail of shadows, the following seems to be the case:
• The trail of reflective light photographic exposure metering starts with the assumption that the "representative" scene:
•• Has an average reflectance of 18% (aha!).
•• Has a maximum reflectance of 100%
But, as "seen" by the exposure meter, and later the camera, this means:
• The average luminance of the scene is 18% of its maximum luminance.
If we follow the trail through the ISO standard for the sensitivity of digital cameras and the ISO standard for photographic exposure meters, and choose an appropriate value for the reflected light metering calibration constant, K, we find that for a metered exposure, the average photometric exposure on the camera sensor will always be (regardless of the scene particulars) about 12.7% of the saturation exposure (the highest exposure at which the imaging system cam properly determine differences in scene luminance—that is, properly record detail).
Now for our "representative" scene, with an average luminance of 18% of its maximum luminance, that means on the sensor the greatest phtometric expose will be about 70.6% of the saturation exposure - that is, almost exactly 1/2 stop short of saturation. That 1/2 stop "clearance" is viewed as "headroom" to accommodate scenes that do not fit the "representative" model.
Note that this only works out this way if we choose the "appropriate" value of K for the exposure meter. There is no "correct" value of K - that can be chosen by the exposure meter manufacture from a range given in the ISO standard to "tune" the meter to what they feel is the best for users.
Best regards,
Doug
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