Doug Kerr
Well-known member
Videographers (especially of the professional persuasion) are often alert to the matter of incident illuminance on the scene (in the sense of in lux).
But even when we actually use incident light metering, we rarely could answer the question, "Well, what was the illuminance on the model's face (or wherever we put our meter)?"
The practical reason is that we generally, in effect, tell the exposure meter what we want it to think is the ISO sensitivity of our film or digital camera, and let it tell us combinations of shutter speed and aperture (as an f-number) that it believes it should suggest.
Many meters don't even have a scale that will indicate measured scene illuminance, even in APEX terms (the quantity "Iv"). (Or if it does - we don't know it. My old Miranda Cadius meter, on the "green" range, reads Iv, but how would you know it?)
But if we want to know for some reason, we can easily "work the exposure equation backwards".
If we are actually using an incident light meter, we can do it this way:
Ei = (224*N^2*T)/S
where:
Ei is the incident illuminance, in lux.
N is the numeric value of the F-number (that is, for f/2.8, N=2.8)
T is the denominator of the shutter speed (that is for a shutter speed of 1/500 sec, T=500).
S is the ISO speed we have set into the meter (the exposure index); for ISO 1600, S=1600.
Now suppose instead we use a reflective light meter (including the exposure control system in our camera). This is of course an approximate process at best. (Since the meter isn't actually "sensing" the incident illuminance, no matter what we know about the calibration of the metering system, we can't actually calculate the incident illuminance - the meter doesn't know it, so how can we reckon it exactly from what the meter tells us?)
Nevertheless, if we adopt some reasonable assumptions (I'll spare you hearing what they are), we can use the equation above for this situation as well.
Here's a numerical example (for either type of metering):
ISO speed : ISO 1600 (S=1600)
Aperture: f/2.8 (N=2.8)
Shutter speed: 1/500 sec (T=500)
Then we calculate that the scene illuminance, Ei, is about 550 lux.
But even when we actually use incident light metering, we rarely could answer the question, "Well, what was the illuminance on the model's face (or wherever we put our meter)?"
The practical reason is that we generally, in effect, tell the exposure meter what we want it to think is the ISO sensitivity of our film or digital camera, and let it tell us combinations of shutter speed and aperture (as an f-number) that it believes it should suggest.
Many meters don't even have a scale that will indicate measured scene illuminance, even in APEX terms (the quantity "Iv"). (Or if it does - we don't know it. My old Miranda Cadius meter, on the "green" range, reads Iv, but how would you know it?)
But if we want to know for some reason, we can easily "work the exposure equation backwards".
If we are actually using an incident light meter, we can do it this way:
Ei = (224*N^2*T)/S
where:
Ei is the incident illuminance, in lux.
N is the numeric value of the F-number (that is, for f/2.8, N=2.8)
T is the denominator of the shutter speed (that is for a shutter speed of 1/500 sec, T=500).
S is the ISO speed we have set into the meter (the exposure index); for ISO 1600, S=1600.
Now suppose instead we use a reflective light meter (including the exposure control system in our camera). This is of course an approximate process at best. (Since the meter isn't actually "sensing" the incident illuminance, no matter what we know about the calibration of the metering system, we can't actually calculate the incident illuminance - the meter doesn't know it, so how can we reckon it exactly from what the meter tells us?)
Nevertheless, if we adopt some reasonable assumptions (I'll spare you hearing what they are), we can use the equation above for this situation as well.
Here's a numerical example (for either type of metering):
ISO speed : ISO 1600 (S=1600)
Aperture: f/2.8 (N=2.8)
Shutter speed: 1/500 sec (T=500)
Then we calculate that the scene illuminance, Ei, is about 550 lux.