A back light illuminates a subject from behind relative to the camera. Depending on its angle and spread, it can produce a rim, edge or hair highlight and help separate the subject from the background.
Position and effect
Moving the source farther around the subject changes how much light wraps onto the visible side. Its height and size affect the highlight on hair, shoulders and clothing. Hair light and rim light describe particular uses; they are not always identical setups.
Controlling spill and flare
Judge the effect through the camera. A source can look modest to the eye yet clip bright hair or reflective fabric. Keep unwanted light off the lens with suitable positioning and flags, and check the full camera move.
Practical example
Against a dark wall, a restrained edge on a dark jacket may provide enough separation. Against a bright window, the background may already separate the subject and a further back light may be unnecessary. Choose the level for the image rather than adding one automatically.
Related terms
Key Light · Fill Light · Hard Light