A term most often associated with the movie industry, but also commonly used in still photography in studio situations or bright light out of doors. A flag is used to block light, create shadow, or shield camera equipment from light sources likely to cause lens flare.
Flags can be any shape or size and may be affixed temporarily to the light itself or free-standing. For high light sources stands and rigs are used to keep a flag in place above the set.
Commercially made flag units are often made of strong, but light, opaque material over a light frame. The lightness of these is important to help with transportation and safety on set (especially as they are often used at high level).
When a photographer holds their hat up to shade the lens and prevent flare they are creating a sort of flag.
Certain types of use of flags are sometimes confused with a gobo. A gobo is more often used to shape light/shadow situations whereas a flag is more often used to create a block on light.
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