Which of the following is primarily used to focus light on a stage?

The ellipsoidal reflector is primarily used to focus light on a stage due to its unique design, which allows it to produce a highly concentrated beam of light. This type of lighting fixture uses an ellipsoidal reflector, which directs the light from the bulb into a focused beam that can be sharply defined or softened by using gobo (pattern) inserts and lenses. This controllability makes it ideal for creating specific lighting effects, shaping light, and focusing on specific areas or performers on stage.

In contrast, a floodlight produces a broader and softer beam of light, making it less suitable for focused applications. Spotlights, while they do allow for focused beams, are often more simplistic and lack the precision of an ellipsoidal reflector in terms of shaping and controlling the light. Backlights, as the name suggests, are used to illuminate the background of a scene, contributing to the overall atmosphere but not specifically for focusing light on subjects in the foreground. Therefore, the ellipsoidal reflector stands out for its precision and versatility in stage lighting design.