Calibrating Light Meters and why it should be done?

[silvan schnelli]

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]

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]

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]

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.

 

 

 

[silvan schnelli]

Steve Yedlin mentions that he first calibrates his light meter to spec before then using compensation to match the incident and spot to 18% grey. 
 

How can this be done, what apparatus would need to be used? I’m pretty sure he calibrates it using known luminance values and the meters in absolute lux/fc and cd/m^2 mode. 
 

My best bet to do this would be to film grey cards, linearise the gamma and see if it lands at 0.18. 

[Mark Kenfield]

On 4/11/2025 at 5:04 PM, silvan schnelli said:

Steve Yedlin mentions that he first calibrates his light meter to spec before then using compensation to match the incident and spot to 18% grey. 
 

How can this be done, what apparatus would need to be used? I’m pretty sure he calibrates it using known luminance values and the meters in absolute lux/fc and cd/m^2 mode. 
 

My best bet to do this would be to film grey cards, linearise the gamma and see if it lands at 0.18. 

The easiest way to calibrate (or at least check) your light meter these days, is to grab a camera, and use it's false-colour mode to check whether your meter is giving you accurate readings.

I do this before any big shoot, and even 13-years on, my trusty, beat-up L758-C is still giving me accurate readings.

[silvan schnelli]

@Mark Kenfield Yeah that’s basically what I did recently. Well I just linearized the images and then calculated how much compensation I needed to add for the spot and incident readings. But as you mentioned these then, as expected, lined up pretty much with the cameras false color readings.

I also now understand Steve Yedlins explanation and reasoning on his website entry. 

Edited May 3 by silvan schnelli