Author Topic: Calibrating light meters for raw (later to be put in BMDfilm color space)  (Read 3610 times)

QuickHitRecord

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A question for those of you who use light meters to shoot ML raw:

If I am calibrating a light meter with an 18% gray card and putting the line on the waveform at about 45%, how would this translate to situations in which I am shooting raw and then later applying BMDFilm colorspace (or one of the LOG 3D LUTs)? Do I still want to set up my meter so that my footage registers at 45% on the waveform AFTER I’ve applied BMDFilm/LOG?

I'd really appreciate any advice.
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aoikonomou

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Struggling with a Seconic light meter as well.Any luck so far?

baldavenger

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If you want to know where 18% reflectance will fall on a BMDFilm Log curve you can have a look at the VFX IO LUTs in Resolve.  Open the Linear to BMDFilm 1D LUT with a text editor that numerically lists the rows (something like BBEdit). It's a 12bit LUT so has 4096 points, and the BMDFilm Log range is listed as -0.0071215555 to 5.7661304310. That range is mapped to the range 0.0 to 1.0 by the LUT. With 5.7661304310 mapped to 1.0 (point 4096), 1 is mapped to point 710, and by referring to the Lookup Table the corresponding new value is 0.6769, which if you apply the LUT to a grey scale ramp you'll notice is where 100% IRE is mapped to.

If you meter to 18% reflectance, that becomes 20 IRE when viewing a waveform (90% reflectance is maximum reflectance so is mapped to 100 IRE, therefore 18% is mapped to 20 IRE). If you want to know how other reflectance values are mapped, then divide 710 by the value and refer to the Lookup table.  20%(18% reflectance) is at point 142, and the corresponding value is 0.393522.

My math might be a bit sketchy, but hopefully you get the gist of it.

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dfort

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You do know that you shouldn't calibrate a light meter to an 18% reflectance, right? I've heard that several times over the years. Here's an article that might clear things up (or muddy it up if your professors insist on 18%.)
http://www.bythom.com/graycards.htm

All of this reminds me of my days in college. We would spend weeks photographing each other holding gray cards, reading negatives with a densitometer and going through boxes of photo paper until we got the "perfect" match to that gray card. Then when I became a "professional" I learned that everybody was bracketing their exposures.

Ok, motion picture is different, you can't bracket but now you've got all sorts of tools to help nail down the "right" exposure but the right exposure is really more of an art than a science.

That said, the exposure meters I see most in use on sets are the Spectra Cine IV-A and the Sekonic L-758Cine.
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