Lens Accessories & Spares - Centre Filters

A centre filter is a circular graduated filter, with a dark spot at the centre that lessens in density to a completely clear part at the edge. They have stepped mounts to a larger filter thread to avoid vignetting.
For critical shots (e.g. with areas of uniform brightness towards
the image corners) the physically inevitable light fall-off
according to the “cos4 law” can be eliminated by using the
neutral gray Rodenstock center filters available for all Apo-
Grandagon and Grandagon-N lenses (see table). A center filter
should always be used if the image circle of a wide angle lens
is used right up to the vicinity of the circumference.
Rodenstock center filters are concentric graduated neutral
gray filters whose density decreases from the center up to the
transparent rim. The fall in density compensates for practically
all the light fall-off to the image edge from a working f-stop
of 16. The exposure must be corrected by 1.5 or 2.5 stops. This
corresponds to a correction of the exposure time by a factor of
3 or 5 respectively (either aperture or exposure time have to be
corrected, not both at the same time).
Apo-Grandagon Filter thread Exp. correction
& Grandagon-N f-stops / time
35 mm f/4.5 E 67/86 +2.5 5×
45 mm f/4.5 E 67/86 +2.5 5×
55 mm f/4.5 E 67/86 +2.5 5×
65 mm f/4.5 E 58/77 +1.5 3×
75 mm f/6.8 * E 58/77 +1.5 3×
75 mm f/4.5 E 67/86 +1.5 3×
90 mm f/6.8 E 67/86 +1.5 3×
90 mm f/4.5 E 82/112 +1.5 3×
115 mm f/6.8 * E 82/112 +1.5 3×
* This lens is no longer produced; however, the matching center
filter is still available for later completion

Here is a list of Schneider and Rodenstock centre filters that are discontinued and no longer available.