One light direction per scene, written down

Why light breaks a cut before a face does

Two shots can contain the same person, the same wardrobe and the same room, and still refuse to cut together because the key light is on the left in one and the right in the other.

Audiences read this instantly and cannot name it. The feeling is “something is off”, and they usually blame the performance or the face. Editors call it continuity, and on a real set an entire department exists to protect it.

You have no such department. You have a line in a text file, and it has to do the same job.

The four properties worth deciding

Decide these once per scene, before you make a single still:

1. Direction. Where is the key coming from: camera left, camera right, behind, above? Write it as a compass or a clock: “key from camera left, roughly 45 degrees, slightly above eye level”.
2. Quality. Hard or soft. Hard light means a sharp-edged shadow, a single small source, high contrast. Soft means a wrapped, gradual shadow from a large source. This is the difference between midday sun and an overcast window, and it changes the whole feel.
3. Colour. Warm or cool, and whether there is a second colour from a practical light in shot: a lamp, a screen, a sign.
4. Contrast ratio. How dark the shadow side is. Low contrast reads as documentary and daylight; high contrast reads as drama and night.

Four short phrases. Put them at the top of your shot list and repeat them in every single still prompt.

Why the prompt words for light actually work

Unlike mood adjectives, light words appeared in captions constantly and precisely: “warm window light from the left”, “overhead fluorescent”, “backlit at golden hour”, “single hard key, deep shadows”, “soft overcast daylight”. Automatic captioners describe light because light is visible.

So this is one of the places where prompting is reliable. Name the direction, the quality and the colour, and you will usually get them.

Motivate the light inside the frame

A shot where you can see why the light is there holds up better than one where it just exists. A window on the left, a lamp on the table, a screen glowing on the face, a doorway behind.

This matters for a specific technical reason as well as an aesthetic one. If a light source is visible in the still, the model has an anchor for it and tends to keep the lighting stable through the clip. If the light has no visible cause, it drifts (the shadow side brightens, the key wanders, the colour creeps warm) because nothing in the picture is holding it in place.

What the model does badly with light

Two honest limits.

It will not change the light convincingly mid-clip. Ask for a lamp being switched on and you generally get a global brightness ramp rather than a light appearing from a position and casting new shadows. There is no light transport being computed; there are plausible frames being sampled. If a shot needs a lighting change, do it as two shots with a cut, or do it in the grade.

Reflections and speculars drift. The bright point on a metal edge or in an eye moves as the camera moves, and getting that right requires knowing where the light and the surface are in three dimensions. The model approximates. On a wet street or a car body this is where the shot gives itself away, and it is a good reason to prefer matte surfaces in anything you need to hold up.

Checking light without expensive tools

You do not need a trained eye, you need a histogram and a side-by-side.

• Open two stills next to each other at the same size. Mismatches in direction are obvious at a glance and invisible from memory. Always compare, never recall.
• DaVinci Resolve’s free tier includes false colour and scopes. Drop two stills on a timeline, switch to false colour, and the exposure difference becomes a flat readable map. GIMP’s histogram will show you a gross brightness mismatch for nothing.
• On a phone, put two frames in a split-screen or a collage app. Crude, and it works.

The grade can rescue colour, not direction

A warm-versus-cool mismatch between two generations is fixable in the edit: a shot-match pass in Resolve pulls one toward the other in a minute. That is a real safety net and you should use it.

What no grade fixes is a shadow falling the wrong way. That is geometry, not colour, and the only remedy is regenerating the still. Which is why the four phrases go in the file before you start, not after.

Today: write your scene’s four light phrases on one line. Put that line into every still prompt for that scene, and put the finished stills side by side before you animate any of them.