Portrait Lighting Ratio Calculator – Key to Fill

A lighting ratio describes the brightness relationship between the main light (Key) and the shadow-filling light (Fill) in a portrait setup. It's typically expressed as Key:Fill — for example, a 2:1 ratio means the Key light is twice as intense as the Fill light. This ratio directly controls the contrast and mood of your portrait: lower ratios produce flatter, softer looks; higher ratios create dramatic shadows and depth. The ratio is measured using a handheld light meter (incident or spot) at the subject's position.

This is a common source of confusion! Key:Fill ratio compares the Key light alone to the Fill light alone. The overall highlight-to-shadow ratio (also called the "total lighting ratio") is (Key+Fill) : Fill — because the highlight side of the face receives both lights, while the shadow side receives only the Fill.

Example: Key = 200 lux, Fill = 100 lux → Key:Fill = 2:1, but the highlight area gets 300 lux (200+100) vs shadow area's 100 lux → overall ratio = 3:1. Both conventions are used in photography literature — our calculator shows both to avoid ambiguity.

Use an incident light meter at the subject's position, pointing toward each light source individually:
1. Turn off the Fill light and meter the Key light alone at the subject's face. Note the reading.
2. Turn off the Key light and meter the Fill light alone at the subject's face. Note the reading.
3. Enter both readings into this calculator.

Pro tip: Shield the meter with your hand from the other light source if you cannot turn lights off individually. For strobes, use a flash meter in corded or cordless mode and fire each light separately.

The 3:1 Key:Fill ratio (which gives a 4:1 overall highlight-to-shadow ratio) is widely considered the classic portrait standard. It provides clear modeling and depth while retaining plenty of shadow detail. Here's a quick guide:
• 1:1 — Flat lighting, used for beauty/cosmetics or high-key looks
• 2:1 — Soft shadows, ideal for corporate headshots and natural-light looks
• 3:1 — Classic portrait modeling, good for general portraiture
• 4:1 — More dramatic, suited for artistic and moody portraits
• 6:1–8:1 — Very dramatic, often used in film noir or character studies
• 16:1 — Extreme contrast; shadows approach black, essentially single-light

EV uses a logarithmic scale (base 2), so a 1 EV difference between Key and Fill means the Key light delivers twice the light — a 2:1 Key:Fill ratio. The relationship is: Ratio = 2^{(EV_Key − EV_Fill)}.

• EV diff 0 → 1:1
• EV diff 1 → 2:1
• EV diff 2 → 4:1
• EV diff 3 → 8:1
• EV diff 4 → 16:1

This makes EV an incredibly intuitive way to think about ratios — each stop of difference doubles (or halves) the ratio. Many studio photographers set their lights by adjusting EV differences directly on their meters.

Yes, absolutely. The lighting ratio is a dimensionless proportion — it doesn't matter whether you measure in lux, foot-candles, or any other unit, as long as both readings use the same unit. 1 foot-candle ≈ 10.764 lux, but since the ratio cancels out the units, the result is identical. Our calculator supports lux, foot-candles, and EV for convenience. Just make sure both Key and Fill are measured in the same unit.

If Fill > Key, your Key:Fill ratio will be less than 1:1 (e.g., 0.5:1). This is unusual in traditional portraiture but can be a creative choice — it produces a reverse modeling effect where shadows appear on the opposite side. More commonly, this indicates that you may have swapped the labels — double-check which light is your main Key light. Our calculator will still compute the ratio correctly either way, and the swap button (⟷) makes it easy to flip them.

Definitely! Cinematographers use lighting ratios extensively to maintain visual consistency across shots and scenes. The same principles apply — a 2:1 or 3:1 Key:Fill ratio is common for interviews and narrative scenes, while higher ratios create tension or drama. For video, ratios are often described using stops (EV difference), as this maps directly to camera exposure. Use our EV mode for quick stop-based calculations.