Highlight headroom is the measure of how much additional dynamic range a given photographic medium (such as film or digital image sensors) has to record the detail within the brightest parts of a scene.
With limited highlight headroom, it will be hard to appreciate the intricate details within the fabric of the dress.
If this bright spot (called a highlight) lies at the edge of the person's outline (from the camera's perspective) then it may end up blending into the cloudy sky if there is insufficient highlight headroom.
This is a particular problem with digital cameras which often have a smaller dynamic range than film.
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