Control the shadow without switching on another lamp
White reflectors, black boards and white walls send existing light along different paths.
What lights the side opposite the key?
Even with one lamp, walls, floors and a pale shirt can fill the darker cheek. Before adjusting the lamp, trace these return paths. A white reflector redirects existing light; distance and angle affect how much returns. A silver surface may create a more directional return, so watch for glare and bright patches.
A black board does not project darkness
Negative fill reduces light that would otherwise return from the room. In a white studio, a black board can restore shape by interrupting unwanted bounce. Keep it out of the key-light path: blocking the key and removing reflected fill are different interventions.
Decide how much of the shadow must remain readable
A straightforward record may need both eyes easy to read; an expressive portrait may leave more of a cheek dark. Decide where attention belongs before adding or removing a reflector. The questions below lead from a visible problem to a specific action rather than to an abstract mood effect.
What is visible in this photograph?
Open the question that matches the problem and trace the returning light.
The opposite cheek is too dark
Return some existing light with a white reflector. Add enough to read the eye and cheek while retaining the key direction.
The face looks flat despite a side key
Inspect white walls and the floor. A black board on the opposite side can reduce unwanted bounce.
The whole face darkened after adding a black board
The board may block the key. Move it out of the path between source and face, then photograph again.