Originally Posted by
Pere Casals
Bill,
First, the calibration of photographic light meters is covered by ISO 2720:1974. Calibration of in camera internal meters is specified by ISO 2721:1982.
Recommended ISO calibration factors spread 1/6 EV, with recommended K in the range 10.6 to 13.4 (cd/m²). A manufacturer is free to use what K he wants, but there is that recommended range.
IMHO with on axis illumination the incident metering and the spot metering on a card should match... The ISO 2720:1974 recommendations are aiming that.
...but we can have practical situations that would make the readings differ because illumination is not like the one in the calibration conditions:
> Light is more or less directional/diffuse, and the imperfect lambertian card reacts different than the imperfect half dome in the incident meter.
> Light is not on axis, for example sunlight illuminating a face in a portrait or a building can come from sun elevated 30º, or 45º or even 90º. Because that Kodak recommends increasing the exposure by 1/2 stop when sun is high in the sky. Again a half dome and the card react different to out of axis illumination.
What I understood from the reading of the ISO norms is that incident light meters are calibrated to a 12% reflectance in the formulas while reflective meters are calibrated to around 18% (16-18%), in that way both kinds of meter deliver the same reading with on axis illumination on a 18% lambertian reflective subject.
So IMHO that 18% vs 12% difference in the calibrations is introduced just to make the incident reading match the spot reading, but this is with on-axis illumination, when illumination is well not on-axis we'll have a difference coming from different reaction to the off-axis light depending on if we read throught a dome or from a naked photocell.
please correct me if I'm wrong...
____________________________
Then we have spectral sensitivity of the sensors, some are spectrally flatter than other... and depending on the light, subject color and filtering...
Bookmarks