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LUT and Exposure


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Hello everyone,

I find myself a bit in the weeds of things and I would greatly appreciate it if someone would help me find my way again.

I recently listened to the Team Deakins podcast with Matthew Libatique, and at one point, he said that there is no relationship anymore between his meter and the camera's exposure setting, unless you use a LUT. 
Now, I am a cinematography student and my program unfortunately does not spend a lot of time on LUT workflows - we shoot on RED Epic Ws with a Helium sensor and use one of RED's LOG-to-Rec709 LUTs. 

I almost entirely rely on my light meter to evaluate exposure because I know the LUT we use tends to crush my blacks and clip my whites before they actually are - the saturation is also a little excessive to my taste. And since I am still wrapping my head around how to build an effective LUT and how to apply it on set, I trust my meter more.

However, when I hear someone like Matthew Libatique talk about the disassociation between light meter readings and camera, I find myself questioning my exposure method, and I feel like I need to start from scratch when it comes to understanding exposure. 

And so, I am wondering if I need to change my approach, or if I can stick with it while considering additional exposure-reading tools, or if my meter is indeed outdated and "all" I need to do is build my own LUT to use in camera to determine my exposure on set.

There is so much contradicting information online and even in cinematography books about this topic, and I was hoping that this forum would be able to help me see the light.

Thank you! 

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I haven’t listened to Mr. Libatique’s episode yet, but I’ll hazard a guess at what he means.

Most people commonly use light meters for relative measurements in f-stops (after inputting ISO and shutter speed values), rather than for absolute measurements in footcandles (incident) or footlamberts (reflected).

This is problematic if the ISO value that you input on the meter doesn’t match the camera’s ISO calibration. Many digital cameras display their gain settings in ISO values, but these are not really standardized and can be whatever the manufacturer want them to be. This is why when you line up different cameras together in a row and set them to 800 ISO, use matching lenses and set the same T-stop, the exposure can be different for each camera.

Light meters are designed to help you reproduce an 18% middle grey accurately. However, there are other considerations for digital camera manufacturers that help sell more cameras - more highlight retention, less noise in low-light, ‘higher’ sensitivity rating - all of which can lead to, shall we say, inaccurate ISO values that don’t match the light meter reading.

So, if you test beforehand to find out the actual sensitivity of your particular camera and take those differences in ISO values into account, then your light meter should work as expected. If not, then you may be surprised with (usually) underexposed results. From what I understand, Mr. Libatique doesn’t have one single digital system that he sticks to, unlike Roger Deakins for example. So I suspect that in his case, relying on a light meter is more trouble than it’s worth.

This also holds true for film stocks, by the way. It’s a good idea to always test a new film stock to find an exposure rating that works for you, rather than relying on the box speed. Though Kodak Vision 3 stocks today are better at dealing with underexposure than ever before, all manufacturer box speeds tend to be overly optimistic. So when in doubt with film negative, overexpose. 

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With regard to LUTs, I’m not sure what he means. At a given setting, the camera captures a certain number of stops over and under middle grey in raw or Log, regardless of what LUT you happen to be viewing with. It might look clipped or crushed on the monitor, but that doesn’t affect what’s being recorded.

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