Rodenstock 67mm Center Filter - When working with ultra-wide-angle lenses, the fall off of illumination towards the corners is sometimes disturbing. For this reason, concentrically graduated Center Filters are available to compensate for this.
For maximum results and definition when using the a center filter, it is necessary to increase the exposure 1.5 stops (3x exposure increase). It is also to be noted that the lens must be stopped down at least two full stops below maximum aperture, since only then can the maximum compensating effect of the center filter be achieved.