This Horseman 67mm 2.5x Center Filter is used 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 have been designed to compensate for this vignetting.
These filters have 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 wave length of the visible spectrum.
In order to avoid over exposure, and with consideration of the exposure latitude of the film emulsions, we have not fully compensated for the vignetting of the lens. It must be remembered that, when working with ultra-wide-angle lenses, one should avoid high-contrast films, under-exposure, and non-uniform field illumination as much as possible.
This center filter is designed to mate with Horseman wide-angle lens units with 67mm filter threads.