Flash coverage & focal length
The larger the focal length figure of lens on a camera, the further away a subject can be photographed to take up the full screen; but the area that can be covered becomes smaller. Conversely, with a smaller focal length figure, closer subjects can be photographed with wider coverage. The flash coverage is the area that the light from the flash at a set intensity or greater can cover evenly, expressed as an angle. In this way, the flash coverage at which you can photograph is determined by the focal length.
By having flash coverage determined in accordance with focal length, flash coverage can be expressed as the figure for focal length.
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