Angle of Flash coverage
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The angle of coverage for even, uniform, edge-to-edge illumination by a speedlight. Usually expressed in terms of focal length. To accomplish such evenness it should always be of a focal length smaller or equal to that of the lens mounted on the camera body while using the flash. When the necessary angle of flash coverage cannot be accomplished through zooming of the head of the speedlight, it is increased with a built-in wide angle adapter that pulls out and flips down in front of the flash head, or with a dome diffuser.
