When you need more than the ND stages 2x, 4x and 8x or want finer graduations, you can select the variable neutral density filter. It consists of two polarization filters whose second filter is a circular polarization filter so that the autofocus and exposure measurement is not impaired. An attenuation by 1.5 to 5 stops (corresponds to the exposure time extension of 2.8x to 32x) can be set continuously over a wide density range by rotating the front polarization filter. A factor of less than 2.8 is not possible as a matter of principle due to the polarization filter transmission which is around 35%.
Variable neutral density filters can be set via 5 stops attenuation (extension factor 32x) up to around 9 stops (more than 400x). However, due to the inherent principle two negative effects then occur (not only with Rodenstock Vario ND filters)
The colour neutrality is gradually lost above a factor of 32x. The reason is that the polarization of the polarization films is much lower for blue-violet light. Blue-violet light is therefore no longer attenuated sufficiently in an approximately crossed position of the polarization filters (i.e. not of equal strength as with other colours) so that a blue-violet colour tinge results. Furthermore, with very wide-angle lenses, disturbing curciform shading occurs close to the maximum position (above around 7 stops or 128x).
Because these very high settings cannot be recommended for the named reasons, the setting scale of the Rodenstock Digital Vario ND only indicates the sensible attenuation range +1.5 to +5 without cruciform shading and without a blue-violet colour tinge.