A lighting ratio describes the difference between the lit side of your subject and the shadow side. It is written as a ratio — 4:1, say — but it is easier to work with in stops, because a stop is what your meter reads and what your camera responds to. The conversion is short, and once you have it, ratios stop being jargon and become a dial you can turn.
What is being compared
The lit side of the face receives the key light plus whatever the fill contributes, because fill is usually broad enough to spill across the whole subject. The shadow side receives fill alone. So the ratio is:
ratio = (key + fill) : fill
This trips people up constantly. If your key meters f/8 and your fill meters f/5.6 on their own, the lit side is not f/8 — it is f/8 plus a bit, because the fill is landing there too. Measured properly, that setup is a 3:1 ratio, not 2:1.
Ratios and stops
Every doubling of the ratio is one stop, because a stop is a doubling of light. That gives you a conversion you can do in your head:
| Ratio | Difference | Character |
|---|---|---|
| 1:1 | 0 stops | Flat. No modelling at all. Beauty, high key, corporate headshots. |
| 2:1 | 1 stop | Gentle. Just enough shape to avoid looking flat. Commercial portraits. |
| 4:1 | 2 stops | The classic portrait ratio. Clear modelling, shadows still hold detail. |
| 8:1 | 3 stops | Dramatic. Shadow side going dark, detail beginning to disappear. |
| 16:1 | 4 stops | Low key. Shadow side essentially black on most sensors. |
Anything past about 16:1 stops being a ratio in any useful sense — the shadow side is simply unlit, and you are lighting with a single source and a lot of negative fill.
How to measure it
Use an incident meter at the subject position, not a reflected reading. Reflected metering measures the subject as well as the light, so a dark jacket and a pale face give you different numbers under identical lighting, which is not what you are trying to find out.
- Turn off the fill. Meter the key from the subject position, dome pointed at the key. Note the reading.
- Turn off the key, turn on the fill. Meter again, dome pointed at the camera. Note the reading.
- The difference in stops is not quite your ratio — add the fill contribution back onto the key side to get the true (key + fill) : fill figure. In practice, for differences of two stops or more, the error is small enough to ignore.
- On set with no meter, a false colour or waveform view on the monitor gives you the same information in a different form.
Distance is a lever, not just power
Light falls off with the square of distance, so moving a fill light is often faster than adjusting it. Pulling a fill from 2 metres to 2.8 metres costs one stop — the same as halving its power — and it also changes the quality of the light, because a source further away is relatively smaller and therefore harder.
That coupling is worth thinking about before you reach for the dimmer. If you want a deeper ratio and softer shadows, dim the fill and leave it close. If you want a deeper ratio and crisper shadow edges, move it back. Two ways to the same number, two different pictures.
Negative fill counts
You can also deepen a ratio by taking light away rather than adding it. A black flag or floppy on the shadow side absorbs the ambient bounce that would otherwise lift the shadows, which is often what is stopping you from getting past 4:1 in a small white room.
In a bright interior with white walls, the room itself is a fill source you did not ask for. Measuring your ratio and finding it stubbornly shallow usually means the walls are winning. Negative fill is the fix, and it costs nothing.
How ratios land on the sensor
A ratio is a contrast decision, and where it lands depends on where you place the key. A 4:1 ratio with the key exposed at middle grey puts the shadow side two stops down, comfortably inside any modern sensor. The same 4:1 with the key placed near clipping puts the shadow side two stops below that, which is still bright. Ratio and exposure placement are independent choices, and confusing them produces images that are technically correctly rated and look nothing like the intent.
For delivery in a log or HDR pipeline you generally have more shadow latitude than the eye needs, so a deep ratio survives the grade. For a straight-to-Rec.709 broadcast delivery, ratios past 8:1 will crush, so the on-set ratio should be shallower than the look you want and the contrast added in the grade.
Common mistakes
- Metering reflected instead of incident. Gives you a subject-dependent number rather than a lighting number.
- Forgetting the fill lands on the key side too. Systematically under-reports your ratio.
- Chasing a number instead of looking. The ratio is a description of what you built, not a target to hit. 4:1 on a broad soft key and 4:1 on a hard small key look nothing alike.
- Ignoring the background. Subject ratio and subject-to-background ratio are separate; a beautifully lit face against an equally bright wall reads as flat regardless of the key-to-fill figure.
Run the numbers
Convert between ratios, stops and meter readings, or work out the fill level you need from light distances.
Related reading
Ratios sit on top of the basic setup covered in Three-Point Lighting Explained, and the falloff behind distance changes is in The Inverse Square Law. For the shadow-edge side of the decision see Hard Light vs Soft Light, and for judging ratios on a monitor rather than a meter, Waveform Monitors and False Color.