Clown said:screw on hoods are mostly non-petal. cuz petal hoods are lens specific.
of course, mathematically and physically speaking, you'd have to end the thread somewhere where it corresponds to the vertical/horizontal angle of view of the lens mounted on the camera. that's why generally, petal hoods are found more as bayonet style, which screw on's are more to fixed focal length lenses, though there's always the exception.