This 67mm Horseman 2.5x Center Filter for SW Series Lenses is used when working with ultra-wide-angle lenses; the falloff of illumination toward the corners is sometimes disturbing. For this reason, concentrically graduated center filters have been designed to compensate for this vignetting. This filter has a transparency which increases gradually from the center to the edges of the filter, where it reaches full transparency. The transmission is independent from the wavelength of the visible spectrum.
It must be remembered that, when working with ultra-wide-angle lenses, one should avoid high-contrast films, underexposure, and non-uniform field illumination as much as possible. This center filter is designed to mate with Horseman wide-angle lenses with 67mm filter threads.