Doug Kerr
Well-known member
Those of you who have ventured into our "theory" room may know that for a week or so I have been struggling to develop a satisfying model of why incident light metering with a "hemispherical receptor" instrument (perhaps the most common type of "serious" incident light meter) should yield an exposure recommendation that is considered to be "very appropriate" over a range of different lighting situations.
I have here brought the discussion out of that musty chamber to seek insight as to one practical impact of the matter.
Don Norwood, the father of the "hemispherical receptor" meter, says, in his definitive patent on the scheme:
At first reading, that sound eminently reasonable. But upon further analysis, it does not really do the trick. In the complex matter of getting the "exposure result" we want with a given lighting situation, it is hard to see why the "total amount of light" that lands on the entire camera-facing surface of the subject should lead to a determination of the optimal exposure. We are after all concerned with the exposure result on individual elements of the subject.
But an interesting glimpse into Norwood's vision is given in an important paper he wrote in 1941 [J SMPE 1941, 36:389-402.] In one discussion, he considers two "clean" lighting situations:
A. Only a single source (floodlamp, perhaps) from alongside the camera.
B. Only a single such source from one aide of the subject.
He notes that the working of a hemispherical-receptor incident light exposure meter (with the receptor always facing the camera, a fundamental tenet of such technique) will, in situation B, recommend an exposure (in the sense of shutter speed and aperture) twice that as in situation A.
I would be interested as to how this resonates with the experience of those of you who do use incident light metering for exposure planning.
Thanks.
Best regards,
Doug
I have here brought the discussion out of that musty chamber to seek insight as to one practical impact of the matter.
Don Norwood, the father of the "hemispherical receptor" meter, says, in his definitive patent on the scheme:
One of the particular objects of the invention is to provide an exposure meter which is substantially uniformly responsive to light incident upon the photographic subject from practically all directions which would result in the refection of light to the camera or other photographic register.
At first reading, that sound eminently reasonable. But upon further analysis, it does not really do the trick. In the complex matter of getting the "exposure result" we want with a given lighting situation, it is hard to see why the "total amount of light" that lands on the entire camera-facing surface of the subject should lead to a determination of the optimal exposure. We are after all concerned with the exposure result on individual elements of the subject.
But an interesting glimpse into Norwood's vision is given in an important paper he wrote in 1941 [J SMPE 1941, 36:389-402.] In one discussion, he considers two "clean" lighting situations:
A. Only a single source (floodlamp, perhaps) from alongside the camera.
B. Only a single such source from one aide of the subject.
He notes that the working of a hemispherical-receptor incident light exposure meter (with the receptor always facing the camera, a fundamental tenet of such technique) will, in situation B, recommend an exposure (in the sense of shutter speed and aperture) twice that as in situation A.
This is in fact exactly what theory would predict.
He then notes that experienced photographers would, by "rule of thumb", often arrive at the same exposure decision: essentially giving the exposure a "one-stop bump" (based on what the light source would otherwise suggest, for a consistent distance from the subject) when the light source is directed from the side.I would be interested as to how this resonates with the experience of those of you who do use incident light metering for exposure planning.
Thanks.
Best regards,
Doug