Lighting ratios in stops

Almost every "the lighting feels wrong" note is really about one number: how much dimmer the fill is than the key. Photographers and cinematographers have measured it in stops for a century, and a stop is a factor of two, because that is how light actually behaves.

The same figure, the same two lights, the same directions. Only the key-to-fill ratio changes: 1:1, 2:1, 4:1, 8:1.

Nothing moves between panels. The lights stay where they are, the shapes are identical, and the mood goes from flat and documentary to hard and dramatic entirely on one multiplier.

Why stops and not percentages

Light is multiplicative, and so is perception of it. Halving the fill is one stop whether you started bright or dim, and it looks like the same size of change either time, which is not true of subtracting a fixed amount. So the dial belongs in powers of two: 

float fillLevel = 1.0 / pow(2.0, stops);
// stops = 0 -> 1.00   flat, no modeling
// stops = 1 -> 0.50   gentle, everyday
// stops = 2 -> 0.25   clearly modeled
// stops = 3 -> 0.125  dramatic
// stops = 4 -> 0.06   near silhouette

Two useful anchors: broadcast and corporate work sits around 2:1, which reads as "well lit and unremarkable". Film noir runs 8:1 and higher, which is the fourth panel and beyond.

The fill has a color, and it is not the key's

Fill is bounce and sky. It has traveled further, hit something, and lost the warmth. Making it a cool blue against a warm key does two things at once: it separates the shadow side by temperature rather than only by value, so the form still reads at high ratios, and it stops the shadow going dead gray.

Matching the fill to the key is one of the most common ways a code-drawn scene ends up looking flat even when the ratio is right.

The rim is not the fill

At 8:1 the shadow side is nearly black, and against a dark background it will merge into it and the silhouette will disappear. That is what a rim light is for, and it is a separate thing from fill: it rides the edge rather than the surface, so it separates the subject from the background without lifting the shadow and destroying the ratio you just set.

Rules of thumb

  1. Set the ratio in stops. It is one number and it does most of the mood.
  2. 2:1 for ordinary, 4:1 for modeled, 8:1 and up for dramatic.
  3. Cool the fill against a warm key so the shadow side separates by temperature too.
  4. Add a rim before lowering the ratio, not after, or the shadow side vanishes.
  5. If it looks flat at the right ratio, the fill is probably the same color as the key.

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