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Translating meter readings from middle grey to the real subject

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Hi all,


I'm not a new filmmaker but am changing the way I work and figured this might be the best section for this question to be posted. 


I’m trying to move away from ‘close enough’ to absolutely nailing things in-camera as much as possible.

Wanted to ask a question about light meter use.

So let’s use this hypothetical - say you are using a camera rated at 800iso. 25fps 180deg shutter speed.


Let’s say shooting Sony S-log3.

You want to shoot a subject at F2.8 and have a key to fill ratio of 2:1 and a key to background ratio of 4:1


·       Key is metered for f2.8

·       Fill is metered for f2

·       Background is metered for f1.4


Now I have a set of meter readings for what ‘correct’ exposure for 18% middle grey is for each part of my scene with those ratios in place. But my subject I want to light to is a human face.

Middle grey in Slog3 is 41% - although most people I know would overexpose that as a matter course to offset Sony’s optimistic camera ratings.


From this point is it just practice and coming up with my own formula to translate those readings to relate to my subject’s skin - for example deciding 0.5 over (or under) middle grey is a good average Caucasian skintone - or is there a more universally accepted standard way of interpreting the meter readings and then applying them, or are you all quite happy using 18% middle grey as being an accurate enough facsimile of the average reflectance of a Caucasian human face which you will then tweak on a per person basis?

I understand the other variables like the need to protect either shadows or highlights in the scene and so shifting exposure or ISO to offset that, but nailing this formula feels like this is where the magic is.


Any guidelines or advice gratefully received.


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I think you need to come up with your own formula. There is no universal standard for exposure metering, even on celluloid film. Some people use a spot meter and a grey card, others only use an incident meter and footcandles, still others rely on a waveform monitor, and some just light by eye. You just have to test, test, test and find what works best for you.

Personally, I would be wary of using reflected readings for setting the stop based on foreground skin tone as a rule, other than as a double check after getting 90% of the lighting in. Otherwise, you risk the background exposure going up and down as you ride the stop based on each person’s skin tone. I find that is more of an ‘available light’ style of shooting, which can look good but will tend to be all over the place if relied upon all the time.

If possible, I prefer to set the exposure for the set/background first and work my way downstage, adjusting the key/fill last by eye. I find it results in more consistent exposures. But of course, in some cases one has no choice but to rely on available light as well. 

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Thanks for that confirmation that people interpreting that meter (or waveform/etc etc) information is kind of what makes the magic.

I've been going by waveforms for over a decade to nail the key and then rough in everything else by eye but wanted to introduce a more scientific and repeatable methodology, especially when having to communicate to any other members of a crew. 


Was using the skintone as an example to simplify everything but really I'm talking about setting exposure from the key - whatever that is hitting -  and working back from that. But absolutely yes once it is set it's set and small variances between characters are something for post to sort, if it needs sorting. 

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One other thing I would like to say - there is no such thing as a universally ‘proper’ exposure unless you’re shooting tests or working in lab-like environments.

What we do is more art than science. You set the target by pre-visualizing what you want the image look like, so ‘nailing’ the exposure is only in relation to what you’re trying to achieve.  Put another way - if it looks good, then it’s right. 

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Absolutely. I guess that's what I am trying to explore the limits of - the point at which art/experience/eye takes over from physics and photometry and the immutable laws of the universe. 🙂


And then finding a way to communicate that to a team.

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Best of luck to you!

One thing you might try is to let your gaffer know roughly how many footcandles you’ll need per scene/location in prep. That’s a level of technical that your gaffer may find useful!

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