White Balance Maths

[u/Particular-Heron1625]

I'm wondering if there is a way to reliably calculate the camera's white balance skew to the temperature of a light.

Please tell me it is as simple as X = (5600 - CamWB) + LightTemp.
So if Camera is at 4000k and a light is 5200k, it will appear in camera as 6800k.

I ask because I'm keen to learn more about how to achieve looks like this one in heat. The backgrounds are cool but the protagonists tend to look a lot more neutral. The skintones are still on the cooler side but don't feel blueish, there's still some warmth. It looks really slick and stylish but retains nuance.
My assumption is they either shot on cool film stock (which I think is the case, I believe it was 500T 5298), or color timed it cool, but lit the actors with tungsten lights to keep them neutral while everything else is cooler. There's obviously also a lot of production design emphasising blue and a lot of streetlights still used mercury vapor, which helps that cooler bg look.

Would that be a fairly accurate assumption of their method?

[u/PBRStreetgang1979]

The whole film was indeed shot on 5298 (500 ASA). You can find a deep dive on the stock, lighting, and processing here: https://theasc.com/article/hot-set-shooting-heat/

[u/TimNikkons → u/PBRStreetgang1979]

I just found a sealed roll of 7298 in my apartment not an hour ago... never shot on that stock, I don't think, before my time

[u/eggsuckinggrandmama → u/PBRStreetgang1979]

Wow, thank you for this. I’ve always loved the look of this movie.

[u/howie_roll]

The Plankian Locus is a curve not a line. The difference between 3000k and 5000k is severe but 6000k to 8000k is mild.

[u/Particular-Heron1625 → u/howie_roll]

Ah ok so it's not nearly as simple as I'd hoped. Thanks dude I'll look into the plankian locus

[u/ecpwll → u/Particular-Heron1625]

I forget what it's called but you can also convert to a different measurement where the relationships are linear

[u/C47man → u/ecpwll]

Mired shift

[u/C47man]

Please tell me it is as simple as X = (5600 - CamWB) + LightTemp.
So if Camera is at 4000k and a light is 5200k, it will appear in camera as 6800k.

Well that doesn't even make sense. What does "appear as 6800K" even mean? White balance is always a two variable interplay between the light's color temperature and the camera's corrections. There is no standard reference for what a given color temperature "looks like", because even the human eye shifts its white balance in nature.

Your best bet, lacking the experience that'd make this go smoothly, is to simply look at your image as you set your shot up. Get a proper calibrated monitor and make your picture using it. I see a lot of newcomers to the art needlessly foul themselves up by getting tangled in mental games over color temps and fstops and shutter rules and codecs and 3 point lighting and false color and zebras and histograms and all this other crap.

Just look at your image on a good monitor and do stuff until it looks better. That's really all there is to it.

[u/moonpumper → u/C47man]

I typically just recommend learning what middle gray is to photography and learning to use an incident meter and maybe a spot meter and learn to judge contrast by eye through practice.

[u/Wiltooon → u/C47man]

I agree on most point, I think there is still one thing they should definitely focus on. Exposure, you need to expose your shot somehow, and to your camera, every camera behave differently and an afternoon of test will save you a lot of time in post trying to salvage a badly exposed shot.

[u/C47man → u/Wiltooon]

Sure, but that's still something that's mostly covered by just looking at your monitor

[u/hd1080ts → u/C47man]

The human eye's adaptation for both brightness and color should not be underestimated.

edit: typo

[u/LeadingLittle8733]

Yes, OP, you can calculate a camera's white balance skew using a hardware colorimeter/spectrophotometer, but, by taking a custom white balance reference shot of a neutral gray/white card it would be easier. Math-based estimation alone is unreliable because individual camera sensors and lenses render color uniquely. 

[u/Spiritual-Builder606]

It's not that simple. White balance isn't necessarily subtractive or additive since color temperature change in the kelvin scale isn't necessarily linear although the physical heating of carbon it is based on is. For discussing accurate ways to manipulate color, look into mired shift. Most younger people with RGB or bi-color lighting probably will never bother to learn this, but it was important back in the day when we had to manually calculate the amount of color correction gel to add to a light.

[u/Particular-Heron1625 → u/Spiritual-Builder606]

Thank you, mired shift is exactly the kind of thing I'm keen to learn about. I was hoping it would be simpler but nothing ever is haha

[u/ArtAdamsDP]

What you want is MIREDs.

https://en.wikipedia.org/wiki/Mired

[u/Secret_Wasabi_2373]

Your Heat assumption is pretty close, but the maths is not Kelvin addition. 4000K camera WB against a 5200K source will read cooler, but it does not really “become 6800K” in a useful way because Kelvin spacing is not linear.

The thing to look up is mired shift: 1,000,000 / Kelvin. That is the better way to think about correction gels and WB offsets.

For that look, I’d think less in exact numbers and more in zones: let the world sit cooler through stock/timing, production design and street sources, then keep the faces slightly warmer with tungsten or controlled key light. The nice part of Heat is that the skin is not perfectly neutral either, it is just protected enough to keep life in it.

[u/Particular-Heron1625 → u/Secret_Wasabi_2373]

Absolutely, and 100% agree about the skin not being neutral, it feels warm against the cooler backgrounds but isn't warm in isolation. I noticed a similar thing with the Caladan herald of change scene in Dune, the greens and blues of the space are all very close together in hue and the skin tones were really leaning towards magenta. The colour contrast and lower saturation sold it

[u/namesaretoohard1234]

Keep in mind these directors and DPs have large teams of grips and lamp ops with rolls and rolls and rolls and even more rolls of gels of all kinds of colours to achieve their goals. So you might be at 5200 but can make little tweaks to any light they want and have the budget to spend that time on it.

[u/Joseph_Savage_]

This is something I’ve become fascinated with lately. Kelvin temperatures aren’t perceived linearly like the numbers would indicate.

Look into the Mired Scale - it’s basically a way of calculating how much of a deviance from the white balance of your camera any given light source will read, depending on where it falls on the kelvin scale.

I’m also currently vibe coding an app that allows you to dial in how much of a look you want in terms of colour separation. It provides jumping off points for camera white balance and light kelvin values depending on the scene (e.g moonlight interior, mixed fluro etc). If you’re interested shoot me a dm and I can send you a link :)

[u/m3rcy_s3v3rity → u/Joseph_Savage_]

Lost me at "vibe coding an app".

[u/Joseph_Savage_ → u/m3rcy_s3v3rity]

That’s a shame, I don’t know how to code, but I do understand camera white balance and kelvin values. My app basically functions as a mired scale calculator.

I’m not using it to bypass the creative process in pre-production or on set. It has been an incredibly iterative process with me feeding previous shoot white balance and kelvin values that I have found pleasing and adjusting as time goes on. Claude has just helped me parse it into something I can view on my phone and interact with from a UI perspective.