How does an incident light meter determine exposure compared with a reflective meter?
Asked 5/27/2023
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I understand that a reflective light meter is calibrated so that whatever it measures is treated as a nominal middle tone. What I’m trying to understand is how an incident meter arrives at an equivalent exposure.
If I meter an 18% gray card with a reflective meter, and then measure the same lighting with an incident meter at the subject position, should both meters give the same exposure? If so, is an incident meter effectively compensating for assumed subject reflectance, or is the diffuser dome itself optically calibrated so the sensor receives the correct amount of light without any special internal calculation?
On combination meters where you only move the white dome to switch modes, does the change happen mainly through the optics of the dome rather than different electronic logic?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
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The idea is quite simple... Add the right amount of plastic.
Here is a simple diagram of a reflective measure.
You have a light source (L) and some rays hit your gray target (C). This absorbs some rays and reflects 18% of the light to the lightmeter which has a hole (B) and a sensor (A)
When you move the plastic dome, position the lightmeter in the place of the gray card, and point it toward the light, the plastic dome reflects some light, disperses some other light which does not enter the hole (B) and, as if it is well designed, will spill the equivalent of 18% of the light it received inside the hole into the sensor.
You can make the dome bigger, smaller add more plastic, or remove the thickness of the dome so the right amount of light enters and gives you a consistent reading, either incident or reflected.
It does not need a switch, only more or less plastic.
Originally by user37321. Source · Licensed CC BY-SA 4.0
user37321
3y ago
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Generated from our catalog & community — verify before relying on it.
Yes—under the same lighting, a reflective reading from an 18% gray card and an incident reading taken at the subject position should be close to the same exposure.
The key point is that an incident meter does not usually "measure the light, then multiply by 0.18" in some special reflective-style calculation. Instead, the white dome/diffuser is part of the calibration. It scatters and attenuates the incoming light so that the sensor receives an amount of light corresponding to the meter’s exposure standard.
So the practical difference is mostly optical:
- reflective metering measures light bouncing off the subject and is affected by subject brightness/tonality
- incident metering measures the light falling on the subject, largely independent of subject reflectance
That’s why incident metering is often more reliable when the subject is unusually dark or bright.
On meters that switch by moving the dome, the mode change is primarily achieved by the diffuser’s position/design rather than by a separate "assume 18%" calculation. The meter is simply calibrated so that each configuration produces the correct exposure recommendation.
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AI3y ago
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