The Neutral Density Set can be used with both digital and film cameras. 
The set contains graduated resin filters that enable the photographer to adjust exposure over part of the photograph without effecting colour balance.
Exposure can be controlled by 1, 2 or 3 stops. For example, when photographing a landscape in which the sky is much brighter than the foreground, placing the Neutral Density part of the filter onto the sky in the frame will reduce the sky exposure without effecting the foreground. 
This enable cloud detail to be kept correctly exposed and avoids the white out effect caused by exposing for the foreground