PLEASE DON'T VOTE ON THIS IMAGE. It is here only to support the article Using the Graduated Density (GD) Filter to Control Lighting in Outdoor Photography. The filter manufacturers offer their filter lines in three different sizes: A, P and X-pro. You select the filter size depending on the focal lengths and physical sizes of the lenses that you use. The P series filters are the most cost effective for the typical SLR camera user with a wide-angle lens or two.