silvan schnelli Posted February 6, 2024 Posted February 6, 2024 I was attempting to re-read Steve Yedlin's #NeryFilmTechSuff on Light Meter Calibration, but still have a lot of questions that made it difficult for me to even understand what was being talked about. Perhaps these questions are addressed in the first version he published, but I cant seem to find it. My question is: Why is he changing the calibration constants from his light meter and how does this differ from the compensation settings (which I think he also changes). I am quite certain I must have misinterpreted many aspects of the text, but this is the first time I've heard of calibrating light meters. I am assuming that changing the calibration constant with respect to the EV formula will result in different calculated/suggested camera settings, but shouldn't doesn't change the illuminance/luminance being measured. Yet I still don't know why one would or maybe more appropriately phrased, should, do this? He does mention something about personal preference, so my best guess is that it might have something to do with that. If it is based on personal preference my next question would be how does one even figure out their preference. I've read that different manufacturers or even light meters have different calibration constants, but I'm assuming all have the same goal of obtaining correct exposure through the calculated readings, so why change this calibration number? Thank you for taking your time reading this question and apologies for the repetitive structuring of it. Steve Yedlin's page that I am referring to: https://www.yedlin.net/NerdyFilmTechStuff/ExposureEquationsAndMeterCalibration.html
silvan schnelli Posted February 28, 2024 Author Posted February 28, 2024 I'll answer my own question in case anyone reading this also wants to know the answer. I am still not certain why Steve Yedlin calibrates his light meters (I'll have to re-read his post most carefully). However, I have found out that from what I have read, ISO values differ slightly from one manufacturer to the other. This is perhaps also another reason why the term Exposure Index is used instead of ISO (besides the fact that a cinema camera's ISO is always fixed). So an ISO of 100 to a Sekonic lightmeter means something different than an ISO of 100 to an ARRI Alexa, which both could differ from what a Sony Venice perceives an ISO of 100 to be. Hence by calibrating our light meter, we can ensure that the EV calculations are calibrated to accommodate the way the camera processes the exposure. This type of calibration hence doesn't alter the Luminance or Illuminance values obtained when measuring Cd/m^2 (foot-lambert) or lux (foot-candle). Difference between Exposure Index (EI) and ISO https://125px.com/docs/techpubs/kodak/cis185-1996_11.pdf
silvan schnelli Posted January 16 Author Posted January 16 To my understanding now, Steve Yedlin calibrates his light meter due to the fact that the incident and spot meter have slightly different figures to what they consider middle grey.
Don H Marks Posted January 16 Posted January 16 Incident meter readings are somewhat of a dark art. Obviously the reading can change dramatically depending on the orientation of the dome. Anyway, the Incident light meter constant "C" as defined by ISO is also somewhat made up by the manufacturer of the meter to give good results. It may not work for everyone. Quote ISO 2720-1974 The constants K and C shall be chosen by statistical analysis of the results of a large number of tests carried out to determine the acceptability to a number of observers, of a number of photographs, for which the exposure was known, obtained under various conditions of subject matter and over a range of luminances. 1
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