Doug Kerr
Well-known member
Don Norwood, in what I will call the "second round" of his work on reflected light exposure metering, as a result of an elaborate series of subjective tests, concluded (and I paraphrase):
For a light source of a given luminous flux density at the subject, the "effectivity" of that source in producing a certain exposure result decrease linearly with the angle at which the source was located, measured from "at the camera". This "effectivity factor" would be by definition 1.0 at with the light at 0°, and would decline to 0.5 with the light at 90°.
Accordingly,. the photographic exposure required to produce a consistent "exposure result" would vary (on a relative basis) from 1.0 with the light at 0° to 2.0 (one stop greater) with the light at 90°.
If an incident light exposure meter had a receptor whose sensitivity decreased linearly with angle, reaching a relative value of 0.5 at an angle of arrival of the light of 90°, then that meter ("aimed at" the camera) would inherently give an appropriate photographic exposure recommendation in a single measurement.
An intimation of this is that if there were two or more light sources, such a meter would in fact "sum" all their effective values, and use that sum as the basis for making a photographic exposure recommendation.
But if there were in fact two light sources (as in the "key-fill" lighting technique that was the primary initial arena of interest in Norwood's exposure metering system), that theoretical result of a "Nowrood" exposure measurement departed from the refult of the "duplex system", previouly often used in this situation, and generally cionsidered to give an "appropriate" result.
Norwood disposed of this by pointing out that for commonly-used ratios of key to fill light "potency", the fill light had little effect on the appropriate photographic exposure.
Best regards,
Doug
For a light source of a given luminous flux density at the subject, the "effectivity" of that source in producing a certain exposure result decrease linearly with the angle at which the source was located, measured from "at the camera". This "effectivity factor" would be by definition 1.0 at with the light at 0°, and would decline to 0.5 with the light at 90°.
Accordingly,. the photographic exposure required to produce a consistent "exposure result" would vary (on a relative basis) from 1.0 with the light at 0° to 2.0 (one stop greater) with the light at 90°.
If an incident light exposure meter had a receptor whose sensitivity decreased linearly with angle, reaching a relative value of 0.5 at an angle of arrival of the light of 90°, then that meter ("aimed at" the camera) would inherently give an appropriate photographic exposure recommendation in a single measurement.
An intimation of this is that if there were two or more light sources, such a meter would in fact "sum" all their effective values, and use that sum as the basis for making a photographic exposure recommendation.
But if there were in fact two light sources (as in the "key-fill" lighting technique that was the primary initial arena of interest in Norwood's exposure metering system), that theoretical result of a "Nowrood" exposure measurement departed from the refult of the "duplex system", previouly often used in this situation, and generally cionsidered to give an "appropriate" result.
Norwood disposed of this by pointing out that for commonly-used ratios of key to fill light "potency", the fill light had little effect on the appropriate photographic exposure.
Best regards,
Doug