Start here: tap the key light on the floor plan and drag it round the subject; watch the key-to-fill ratio change.Keys: 1–5 pick a light · O on/off · E expose · Ctrl+Z undo · D demo
Floor plan · top view · 0.5 m grid
Pattern –Key:fill –
Camera view

Challenge: match the reference

Recreate this portrait by moving and adjusting the lights. Similarity is scored live, pixel by pixel; reach 92 %.

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Light meter at the subject

Incident readings: the light arriving at the face, measured with the meter pointed at the camera (the meter uses C ≈ 250, the flat-receptor constant; dome meters use about 330, roughly ⅓ stop different). lx = lux (brightness of light on a surface). One stop (st) = twice or half the light. “Needs” is the aperture for ISO 100 at 1/50 s.

Key + fill
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Lighting ratio
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Key vs fill
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Exposure
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Three-point lighting

The standard interview and portrait setup uses three roles. The key light is the main, brightest source; it sets the direction of the shadows and the mood. The fill light, usually on the opposite side of the camera and softer, lifts the shadows the key creates; its strength relative to the key sets the contrast. The back light (hair or rim light) sits behind the subject, opposite the camera, and draws a bright outline that separates the subject from the background. A fourth background light controls the backdrop independently, and a kicker (a low rim light from behind and to the side) adds a highlight along the cheek or jaw.

The inverse square law

A small source radiates its intensity $I$ (candela) into a sphere, so the illuminance on a surface at distance $d$ facing the light is

$$E = \frac{I}{d^2}\cos\theta \quad\text{(lux)}$$

Photographers count light in stops: one stop is a doubling or halving. Double the distance and the light falls to a quarter: two stops. Moving a light from 1 m to 1.4 m costs one stop. Close lights also fall off quickly across the subject (the far ear receives noticeably less than the near cheek), while a distant light gives even illumination. Large sources (softboxes) only behave as points when you are a few times their size away, but the law is a good guide.

Exposure from illuminance

An incident light meter converts illuminance to exposure with the calibration constant $C \approx 250$:

$$\frac{N^2}{t} = \frac{E\,S}{C}\qquad\Longrightarrow\qquad N = \sqrt{\frac{E\,t\,S}{C}},\qquad \mathrm{EV}_{100} = \log_2\frac{E}{2.5}$$

At 1/50 s (25 fps video) and ISO 100, 800 lux needs $N = \sqrt{800\times0.02\times100/250} = 2.5$, close to f/2.8.

Lighting ratio

Contrast is described by the lighting ratio: illuminance on the lit side (key + fill) to the shadow side (fill only), $R = (E_K + E_F)/E_F$, often quoted in stops, $\log_2 R$. 2:1 (1 stop) is flat and friendly (news, corporate); 3:1 to 4:1 (1.6–2 stops) is classic portraiture; 8:1 (3 stops) and beyond is dramatic, low-key work.

Hard and soft light

Softness depends on the apparent size of the source as seen from the subject, i.e. its angular size $\alpha \approx D/d$. A bare bulb or a distant sun is small and gives crisp shadow edges, sharp specular highlights and texture. A softbox, umbrella or bounce is large and gives wide, gradual shadow transitions (penumbra) and broad highlights. Moving a softbox closer makes the light softer (larger apparent size) and brighter, but with more falloff. In this simulator the penumbra width and the size of the highlights come from $\alpha$, computed live.

Classic portrait patterns (named after the nose shadow)

  • Butterfly / Paramount: key in front of the face, high above the camera; a small butterfly-shaped shadow under the nose. Glamour and beauty.
  • Loop: key about 15–40° to the side and a little above eye level; the nose shadow makes a small loop on the cheek that does not touch the cheek shadow.
  • Rembrandt: key about 40–60° to the side and 25–50° above the eyes; the nose and cheek shadows join, leaving a small triangle of light under the far eye.
  • Split: key at 90°; half the face lit, half in shadow.
  • Broad vs short: with the face turned, broad lighting lights the side of the face turned towards the camera (widens the face), short lighting lights the side turned away (slims it and adds depth).
  • High-key: bright, low-contrast, white background lit about 1–2 stops over the subject. Low-key: dark background, strong contrast, rim light for separation.

Colour temperature and colour rendering

Light sources have a correlated colour temperature: tungsten ≈ 3200 K, daylight and most studio LEDs ≈ 5600 K. Mixing sources of different temperature produces colour casts unless corrected with gels (CTO warms a daylight source towards tungsten, CTB cools a tungsten source towards daylight). The camera’s white balance should match the key light. CRI (Colour Rendering Index, max 100) and the newer TLCI measure how faithfully a source renders colours, particularly skin tones; for broadcast use CRI/TLCI ≥ 90. Cheap LEDs with spiky spectra can make skin look green or grey even when the colour temperature is correct.

Model notes: the bust is a stylised signed-distance sculpture rendered with Lambert diffuse light (with a wrap term for large sources), Blinn–Phong highlights, ambient occlusion and soft shadows traced through the depth map; the view is orthographic, light falloff is exact inverse square from the light centre, and modifiers are characterised by size, efficiency and beam spread. Skin rendering is simplified (no sub-surface scattering).

1. Inverse square in stops

A key light gives 1600 lux at 1 m. At 2 m: $1600/2^2 = 400$ lux, a quarter, $\log_2 4 = 2$ stops less. At 1.41 m: $1600/2 = 800$ lux, one stop less. Move the key on the floor plan and watch its lux reading follow $1/d^2$.

2. From lux to f-number

Key + fill = 1000 lux, video at 1/50 s, ISO 100: $N = \sqrt{1000\times 0.02\times 100/250} = \sqrt{8} = 2.83$, so f/2.8. At ISO 400 you could use f/5.6 (two stops).

3. Setting a 3:1 ratio

Key alone measures 1000 lux at the face. For a 3:1 ratio $(E_K+E_F)/E_F = 3 \Rightarrow E_F = E_K/2 = 500$ lux. If the fill gives 1500 lux at 1 m, place it at $d = \sqrt{1500/500} = 1.73$ m, or keep it where it is and reduce its output by $\log_2(1500/500)$ ≈ 1.6 stops.

4. Apparent size and softness

A 90 cm softbox at 1.2 m subtends $\alpha \approx 0.9/1.2 = 0.75$ rad (43°): very soft. At 3 m it subtends 0.3 rad (17°): noticeably harder, and the illuminance falls by $(3/1.2)^2 = 6.25\times$, 2.6 stops.

5. Gels and white balance

Camera at 5600 K. A tungsten 3200 K lamp renders orange. Going from 3200 K to 5600 K needs a shift of $10^6/5600 - 10^6/3200 = 178.6 - 312.5 = -134$ mired (micro reciprocal degrees; negative means bluer). A full CTB gel gives about −131 mired, so it converts 3200 K to roughly 5500–5600 K, at the cost of about 1.5–2 stops of light.

Challenge: match the reference look

The reference challenge sits directly under the camera view: pick one of seven reference portraits and recreate it by moving, switching and adjusting the lights (and head turn, backdrop and exposure). The score compares the two images pixel by pixel (brightness and colour), live. Go to the challenge.

More challenges (auto-checked)

A check that a freshly loaded preset already meets (or that Auto-expose sets for you) only counts once you have built it yourself: change the lights, then bring them back into range.

    Experiments

    1. Inverse square. Load Rembrandt, select the key and drag it from about 1 m to 2 m from the subject: the meter drops by about 2 stops, and the falloff across the face becomes gentler.
    2. Hard vs soft. With the same position, switch the key from Bare bulb to Large octabox. Watch the nose shadow edge and the highlight on the forehead spread out.
    3. Broad vs short. Turn the head 25° and move the key from one side to the other.
    4. Fill and contrast. Lower the fill output in 1-stop steps from a 2:1 to an 8:1 ratio.
    5. Colour mixing. Set the fill to 3200 K with the camera at 5600 K: the shadows turn orange. Then switch the camera to tungsten white balance.
    6. Separation. Turn the back light on and off on the black backdrop.