A hard edge is used when there is an abrupt change in brightness, such as a field with a horizon to a bright sky. A soft edge is a wider, smoother change from light to dark. This is used when the light and dark portions are not distinctly separated, such as a mountain and sky. A soft edge filter is less noticeable than a hard edge. It also has the benefit of making the sky more intense, darkening the sky the closer on the top.