What is the technique called when an actor determines the focus of a beam of light by moving their hand in and out of it?

The technique in question is known as flagging. This term refers to the practice used in theatre lighting where an actor or technician assesses the focus of a beam of light. By moving their hand in and out of the light, they can visually observe how the light interacts with surfaces and determine whether it's correctly placed for the intended effect. This process is crucial in ensuring that the lighting contributes effectively to the performance atmosphere and highlights the actors and scenic elements accurately.

Flagging helps in controlling how light falls and helps identify any unwanted spill or shadows that might affect the scene. This technique is particularly useful during technical rehearsals when adjustments to lighting are frequently made. Understanding this concept allows actors and crew members to collaborate effectively, ensuring that they create the desired visual storytelling elements through lighting design.