Accessories: Center filters
For critical shots (e.g. with areas of uniform brightness towards the image corners) with lenses with an extremely large image angle, the physically inevitable light fall-off can become a disturbing element. This can be avoided by using the Rodenstock center filters recommended in any case the image circle is used right up to its margin. The Rodenstock center filters are available for the Apo-Grandagon and the Grandagon-N lenses of all focal lengths.
Rodenstock center filters are virtually neutral gray filters whose density decreases from the center up to the transparent rim. From the recommended working aperture the fall in density compensates for practically all the light fall-off to the image edge. This corresponds to the light fall-off of lenses with a larger focal length, which does not represent a disturbing element. Because of the filter absorption in the image center, the exposure time must be extended by the factor of 5 for the Apo-Grandagon and by the factor of 3 for the Grandagon-N. Alternatively the aperture can be opened by 1.5 or 2.5 stops.