In theatre, focusing is the aiming, shuttering and coloring of lighting instruments. With conventional lights focusing is done manually and consists of pointing the light at a specific area of the stage as determined by the lighting designer, adjusting for a hard or soft edge to the beam, and in the case of ellipsoidals, shaping the beam through the use of shutters and gobos.
In the case of moving lights, focusing is done through a computerized light board.
This process is usually done after the set as been loaded in, but before the actors take the stage. Depending on the size of the show this process can take a few hours to a few weeks.