Doug Kerr
Well-known member
As I have studied the "Norwood concept" of incident light exposure metering (which is practiced today in almost all serious photographic incident light exposure meters - the hemispherical dome is its hallmark), I had remained baffled by one concept.
I became well familiar with the precise way such a meter would respond to incident light, and with many implications of that.
But what was missing for me is this: Why does the "reading" of such a meter lead to a photographic exposure that, in some way, is "very appropriate" over a range of lighting situations. This question of course leads to a nightmare web of underlying questions, such as "what about a certain photographic result makes us consider it 'very appropriate' from an exposure standpoint."
It turns out that my error was in fact to try to consider the "general case". I now realize that Norwood's work largely focused on a particular real situation he, as a cinematographer, regularly faced: the use of a "key plus fill" lighting setup.
In a 1950 paper, Norwood identifies this context, and (by way of describing the premise of a test program he described) essentially articulated the real issue in this matter, which I will state thus:
Norwood reports a test series to investigate the relationship between the placement angle of the key light and the photographic exposure that would be needed to give an image that would be judged as "equally well exposed" as an image shot with a "head-on" key light and a certain reference exposure.
This in fact led to a function of angle that describes the "relative effectiveness" of a key light of a certain potency directed from a certain angle (amazingly, it is precisely linear with angle) [note 1]. Norwood then points out that if the response of an exposure meter followed that same function of the angle of incidence of the light, then the indication of that meter will, for any placement of the key light, recommend the exposure that will lead to an image that would be judged "as properly exposed as the reference image" (with a head-on key light).
He then goes on to point out that this is in fact exactly the sensitivity function of a hemispherical receptor meter. [note 2]
Thus we see that the use of a hemispherical receptor meter would be expected to give us exposure recommendations, for various key light positions, that lead to exposure results that typical viewers would judge as consistent in terms of "proper exposure".
Very nice.
What about other complex lighting situations? Who knows.
Notes
[1] It is very suspicious that the results of this highly subjective comparison study gives a function of effective influence of a key light versus angle of placement that is precisely linear.
[2] In fact what he cites for the sensitivity pattern of a hemispherical receptor meter is not quite what is typically considered to be the sensitivity pattern of a hemispherical receptor meter. (He cites a linear function for his meter design, whereas it is generally considered that a hemispherical receptor meter would have approximately a cardioid pattern.)
Comment
It is amazing that the peg here fits so precisely in the hole, and I see evidence that both the peg and the hole may have been shaved to make that so.
Thais is not to say that the concept I describe above is not valid. It makes sense to me. And the issue is not one that could have any "precise" conclusions.
I'm just always suspicious of studies that seek to show that a certain change to a telecom system will about double its user satisfaction, and in fact ends up with a measured improvement factor of 2.00000000.
Best regards,
Doug
I became well familiar with the precise way such a meter would respond to incident light, and with many implications of that.
But what was missing for me is this: Why does the "reading" of such a meter lead to a photographic exposure that, in some way, is "very appropriate" over a range of lighting situations. This question of course leads to a nightmare web of underlying questions, such as "what about a certain photographic result makes us consider it 'very appropriate' from an exposure standpoint."
It turns out that my error was in fact to try to consider the "general case". I now realize that Norwood's work largely focused on a particular real situation he, as a cinematographer, regularly faced: the use of a "key plus fill" lighting setup.
In a 1950 paper, Norwood identifies this context, and (by way of describing the premise of a test program he described) essentially articulated the real issue in this matter, which I will state thus:
Consider studio light with a combination of a key light and fill lighting, and consider a range of specific setups where the key light (or a certain consistent potency) aims at the subject from different angles (head-on, from 45° to one side or the other, etc.)
What kind of exposure meter will, over all of those variations, recommend a photographic exposure that will lead to images that will be adjudged by a typical viewer as "equally appropriately exposed".
What kind of exposure meter will, over all of those variations, recommend a photographic exposure that will lead to images that will be adjudged by a typical viewer as "equally appropriately exposed".
Norwood reports a test series to investigate the relationship between the placement angle of the key light and the photographic exposure that would be needed to give an image that would be judged as "equally well exposed" as an image shot with a "head-on" key light and a certain reference exposure.
This in fact led to a function of angle that describes the "relative effectiveness" of a key light of a certain potency directed from a certain angle (amazingly, it is precisely linear with angle) [note 1]. Norwood then points out that if the response of an exposure meter followed that same function of the angle of incidence of the light, then the indication of that meter will, for any placement of the key light, recommend the exposure that will lead to an image that would be judged "as properly exposed as the reference image" (with a head-on key light).
He then goes on to point out that this is in fact exactly the sensitivity function of a hemispherical receptor meter. [note 2]
Thus we see that the use of a hemispherical receptor meter would be expected to give us exposure recommendations, for various key light positions, that lead to exposure results that typical viewers would judge as consistent in terms of "proper exposure".
Very nice.
What about other complex lighting situations? Who knows.
Notes
[1] It is very suspicious that the results of this highly subjective comparison study gives a function of effective influence of a key light versus angle of placement that is precisely linear.
[2] In fact what he cites for the sensitivity pattern of a hemispherical receptor meter is not quite what is typically considered to be the sensitivity pattern of a hemispherical receptor meter. (He cites a linear function for his meter design, whereas it is generally considered that a hemispherical receptor meter would have approximately a cardioid pattern.)
Comment
It is amazing that the peg here fits so precisely in the hole, and I see evidence that both the peg and the hole may have been shaved to make that so.
Thais is not to say that the concept I describe above is not valid. It makes sense to me. And the issue is not one that could have any "precise" conclusions.
I'm just always suspicious of studies that seek to show that a certain change to a telecom system will about double its user satisfaction, and in fact ends up with a measured improvement factor of 2.00000000.
Best regards,
Doug