This is the 95mm Center Filter (#4a) from Schneider Optics. 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 wavelength 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. With the consideration of these three factors, the use of Center Filters in combination with the Schneider Super-Angulon (and Super-Symmar XL) lenses can open new areas of wide-angle photography in black-and-white and color.
For maximum results and definition when using this Schneider center filter, 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. This filter has a front accessory thread of 112mm and uses a 115mm push-on lens cap (SCLC115).