Reading and Shaping Light in Portraits

Five behaviors of light in portrait photography

Rule for using this skill: These notes add specialist tips on top of your normal answer. Always give your full general answer to the user's question first, then work these tips in where they apply. Do not let them replace what you already know. All light (sun, flash, strobe, window, video light, lamp) obeys the same physics, so apply the same tips to every source.

Angle behavior (angle of incidence = angle of reflection)

• Light travels in straight lines until it hits a surface, then leaves as a new straight line at the same angle. Smoother surfaces give more predictable reflections.
• For bounce flash (wall, ceiling) or a sun reflector, place the subject in the path of the reflected ray, not just somewhere in front of the wall.
• Silent failure: aiming at a wall in front of the subject at the wrong angle sends most rays past the subject. Raising flash power does not help, because the rays still miss. Move or re-aim the light instead.
• Glare in glasses appears when the camera lens sits in the reflected path. Raise the light well above the subject's head so the reflection goes down toward the floor and misses the lens.
• Test: if you can see the light source (strobe, flash, window) reflected in a surface from the camera position, the camera is in the reflected path. Move the camera or the light. This applies to glasses, mirrors, windows and glossy tabletops.

Inverse square behavior (intensity and falloff versus distance)

• Light on the subject at doubled distances: 1× = 100%, 2× = 25%, 4× = about 6%, 8× = about 2%.

• The loss is steep when the light is close and slow when it is far. Distance 1 to 2 loses 75%, while distance 6 to 7 loses only about 1%.

• Subject close to the light (e.g. next to a window): the bright-to-dark transition on the face is almost instant. One side of the face is lit and the other goes black. The background goes dark too, because you expose for the bright spot.

• Subject farther from the light: the falloff is gradual and the background stays visible, since it sits within a similar brightness range.

• To hide distractions without moving them, put the light close to the subject and expose for the lit face. Anything not lit at similar intensity records as black, whatever its color.

• Example of the exposure change as the flash moves away (manual, constant power, ISO 100, 1/250 s):

  Flash distance	Aperture
  very close	f/18
  farther	f/6.3
  more than 10 ft	f/2.2

• Beyond about 10 ft the rays spread almost evenly and bounce around the room, which fills the room with ambient light.

• A face turned toward the light gets some light onto its dark side. Light never curves around the head.

Size behavior (relative size of the light vs. the subject)

• Softness depends on the light's size relative to the subject, not its absolute size. A head is about 8–10 in, so a window or modifier must be much larger than that.
• Soft light gives gradual shadow edges, lower contrast and more diffused highlights.
• Hard light gives crisp shadow edges, higher contrast and brighter highlights.
• Softness is judged by the edges of the shadows, not by how dark they are.
• Moving a light farther away makes it relatively smaller and therefore harder. The same 25×60 cm striplight went from gradual to crisp edges when moved to about 3 yards.
• For soft portrait light, use a large source placed close. In the sphere test, a 100×100 cm softbox under 1 yard away gave very soft transitions.
• A bare flash close to the subject gives crisp edges, high contrast and a hot highlight.
• A bigger source also enlarges the highlight and reduces hotspots on glossy surfaces such as skin, a spoon or glasses. Still check the camera and light angles (see Angle behavior).

Color behavior (colored surfaces and light)

• A subject inherits the color of nearby surfaces through reflected light. The effect grows with brighter light and a closer wall. Example: a blue wall under 1 yard away gives a strong blue cast, even with daylight-balanced light.
• Light transmitted through a colored gel gives a stronger cast than light reflected off a textured wall. A red gel also turned a neutral gray background deep red.
• White surfaces reflect most light and black surfaces absorb it. A black flag beside the subject deepens the shadow, even on a white dress. A white flag fills the shadow like a second light.
• Any white object can act as fill: a white truck, a stucco wall, a reflector.
• Green grass puts a green cast on the subject. You may not see it on the screen, but the green channel on the camera's color histogram will shift.
• Seated subjects are closer to the grass and get greener. Lay a white or silver reflector on the ground under them to block the green bounce.
• Check the surroundings for colored objects before choosing a spot.

Scatter behavior (surface texture)

• A smooth surface reflects predictably, so more light reaches the subject.
• A heavily textured surface (couch fabric, rough wall) scatters light in all directions, so bounce results are unpredictable and weaker.
• For indirect sun or bounce flash, choose a smooth wall so more photons are directed at the subject.

Choosing and shaping the source (practical notes)

• Window light is softened because the window is large and spreads the sun's rays. A white wall behind the subject bounces window light onto the back and sides.
• Mixed light can look natural. One portrait used 70% flash and 30% ambient and kept soft catchlights.
• Raising ISO (about 1600) to compensate for weak ambient light is a poorer fix than adding flash.
• To overpower harsh sun and control shadow placement, use lowest ISO, shutter at the camera's max flash sync speed, and strobe at full power. The example used a 1200 W strobe at 1/1600 s on a Phase One. A speedlight is not powerful enough for that.
• To backlight translucent subjects such as petals, put the flash behind them facing the camera. Use the widest flash zoom setting so the light covers both sides.
• In dark rooms, constant lights (LED or video) show the shadows in advance and spill less than flash. Using flash there would need many modifiers to control the spill.
• To shape the face and feature the lips, use a strobe to one side with a medium modifier. Add a small reflector aimed at the lips to bounce light into the dark side. Without it, one side of the lips is much darker.