This is a center filter used to correct illumination falloff on the Super Angulon Classic 90/6.8 wide angle lens. It has a filter factor of 3x in the center (1.5 stops) and is clear at the edges. The mounting thread is 82mm x 0.75 and additional filters can be attached in front, using the 105mm x 1.00 accessory thread.
When working with ultra-wide-angle lenses, the falloff 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 of the wavelength of the visible spectrum. In order to avoid overexposure 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, underexposure, and non-uniform field illumination as much as possible. With the consideration of these three factors, the use of Center Filters in combination with the Schneider Super-Angulon can open new areas of wide-angle photography in black-and-white and color. For maximum results and definition when using the Schneider Center Filters, it is necessary to increase the exposure 1.5 stops. 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.