Sony HVL-F42AM Operating Instructions Manual page 25

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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