How do you white balance on set?

Twenty minutes to the first shot. There are two big bounce boards frame left, a warm lamp behind a tinsel curtain, teal practicals glowing along the far wall and a party light turning over all of it. The DP asks what to set the camera to.

The answer is that it depends where the talent stands. White balance is set for the light that lands on the face, and on a set like this every mark gets a different mix.

Read the light where the face will be

Walk the card to the mark. Hold it at chin height, turned the way the face will turn, so it catches the key and the fill in the same mix the skin will. If you hold it flat to the key you read the key alone, and the fill side of the face can be a long way off.

Keep your own shadow off it, and keep the frame full of card. Anything else in the frame, a bright wall, a warm floor, the talent's red jacket, pulls the reading toward its own color. That is why the card is gray: it is made to be neutral, so whatever color it shows is the light's. A white wall can do the job in a pinch, but walls are rarely as white as they look.

Set kelvin and tint

Kelvin is the warm to cool part of the color, the line that runs from candle to tungsten to daylight to open shade. Tint is the other axis, green to magenta. Tungsten lamps and the sun sit almost on the kelvin line, so on those you can often get away with kelvin alone. LEDs, tubes and mixed spaces drift off it, and that is where tint earns its place.

Most cinema cameras take both as numbers. Many can also white balance off a card with a button, and that works, but a number you dial in can be written on the camera report, repeated tomorrow on the pickup day and handed to the colorist. That is the case for reading the light rather than letting the camera guess.

One caution on tint. Every camera maker scales its tint control differently, so a meter's tint figure is a direction and a rough size, never a number to copy straight into the camera. Use it to see which way the light leans and by about how much, then balance off the card or check a gray card on the monitor.

In mixed light, decide what is white

On a set with several sources you will never make every one neutral at once, and you would not want to. Balance for the light on the face, then look at what everything else does relative to it. A practical reading warmer than the key usually reads as a lamp, which is the point of it. A window reading much cooler reads as daylight outside.

The question is how far apart they are, and kelvin is a poor ruler for that. The same 500 K gap is a big shift at the warm end and a small one at the cool end. Gels and meters talk in mired, a million divided by the kelvin, because a mired step looks the same size wherever it happens. Two lights within about 5 mired of each other read as the same light. The further past that, the more it shows on skin.

Where common sources sit

Typical ratings and published figures. Real lamps drift with age, dimming and voltage, and daylight moves all day, which is why you read them.

A worked example: a split balance

A late-afternoon interior with a window in the back of shot and tungsten lamps in the room. The DP wants the window a little blue and the lamps a little warm, with the face in between, so the camera goes to 4300 K, a common split between 3200 K and 5600 K. Here is what every other light then does on camera, and the sheet that would pull each one back to neutral if you wanted it to.

With the camera at 4300 K, how far off each light reads

Gap is the light's mired minus the camera's: a plus is warmer than white, a minus cooler. Worked out with the site's port of the app's gel search, LEE book.

Neither side is wrong: a tungsten lamp 80 mired warm of the camera reads as a lamp, and a window 54 mired cool reads as daylight outside. What the table tells you is how strong each will look, and what to do if one is too much. Half CTB on the tungsten key brings the face to neutral; an eighth or two of CTO on the window calms it without losing the blue.

Doing it with Cutvey Meter

The meter opens on AWB, which reads the whole frame the way the camera would. Tap the reading to switch to GRAY CARD, then fill the frame with the card, as the app asks you to. Hold steady while the reading settles and watch the badge. HIGH means the reading is steady and clean; MEDIUM or LOW means something is holding it back, like exposure well off, clipped highlights or a light that flickers.

Tap LOG and the reading is kept with the frame you were pointing at, the lens and the exposure. On a light you do not trust, hold LOG for a RAW reading, measured from the sensor's own pixels, and compare it with the camera's estimate. If the light flickers, or the card is lit by two colors at once, the app says DIFFICULT SOURCE: treat that number as a place to start, and check a gray card on the camera.

To set a split balance like the one above, open REF and type the target, 4300 K, then point at each light in turn. The card shows the gap in kelvin, mired and tint, and the gel that closes it.

What color is that light?Common lamps and daylight in kelvin and in mired

Read again when anything moves

A white balance is only true for the light you read. The sun moves through a window over the afternoon. A tungsten lamp dimmed down goes warmer. Someone swaps a bulb in a practical between setups. None of it is dramatic on its own, and all of it shows up when the shots are cut together.

So read at every new setup, and after any change to the lamps. Write the kelvin and tint on the camera report next to the shot. When the edit needs a pickup three weeks later, the numbers are there, and so is the frame they came from.