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Incident meters make the assumption that the subject is three dimensional and that light is reflected toward the cameras from it, in proportion to the visual tone.
It is not in the least interested in "How much light is reflected". It does not and can not measure this.
The Domes of incident meters were originally called "Heliospheres" which gives a clearer idea of their purpose.
Imagine a subject totally covered under a giant "Heliosphere" then bring up a side light.
You would note that Half the sphere was lit in a graduated fashion The side of the subject nearest the light would be lit full blast... but the front of the subject would only be lit by a fraction of that.
As you bring up more lights, so the Dome (and subject) would be lit variously by each light
The Dome on the meter works in just that way, when pointed at the camera each part of it is lit variously and in proportion to the lights hitting it. This light continues on to the Photocell which measures the "Total effective light falling on it, and that is available available to illuminate a 3D subject"
This measurement becomes a proxy for a perfectly reflective 3D subject and in proportion to its visible tones.
As you can see the meter would not give the same reading if it was pointed toward the light, as all the dome would be illuminated. Representing parts of the subject the camera would never see. It would also fail to achieve the the shot to shot standard required for studio or film work.
Back lights and hair lights tend to produce spectral highlights that will burn out, what ever the exposure set.
It is better to take readings with those turned off or ignored, and adjust those lights Visually or by ratio. The important tone and highlights will be maintained.
It is not in the least interested in "How much light is reflected". It does not and can not measure this.
The Domes of incident meters were originally called "Heliospheres" which gives a clearer idea of their purpose.
Imagine a subject totally covered under a giant "Heliosphere" then bring up a side light.
You would note that Half the sphere was lit in a graduated fashion The side of the subject nearest the light would be lit full blast... but the front of the subject would only be lit by a fraction of that.
As you bring up more lights, so the Dome (and subject) would be lit variously by each light
The Dome on the meter works in just that way, when pointed at the camera each part of it is lit variously and in proportion to the lights hitting it. This light continues on to the Photocell which measures the "Total effective light falling on it, and that is available available to illuminate a 3D subject"
This measurement becomes a proxy for a perfectly reflective 3D subject and in proportion to its visible tones.
As you can see the meter would not give the same reading if it was pointed toward the light, as all the dome would be illuminated. Representing parts of the subject the camera would never see. It would also fail to achieve the the shot to shot standard required for studio or film work.
Back lights and hair lights tend to produce spectral highlights that will burn out, what ever the exposure set.
It is better to take readings with those turned off or ignored, and adjust those lights Visually or by ratio. The important tone and highlights will be maintained.
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