Many wide angle lenses exhibit fall-off (darkening) at the edges. This can be improved with a center filter which has a progression from dark grey in the center to clear at the edges. The Heliopan wide angle center filters are available in two densities: ND-0.45 (filter factor 3x, or +1.5 stops) and ND-0.9 (filter factor 8x, +3 stops).
Most large format lens manufacturers make center filters for their wide angle lenses. Heliopan offers an economical alternative for lenses requiring a 3x filter. The 8x version is unique and the most powerful on the market.