In Ansel Adams book 'The Negative', he goes into a bit if detail regarding the advantages of pre-exposing both black and white and transparency film, primarily where increased shadow detail is neccessary.
He mentions the possible use of a color compensating filter sandwiched between two opaque sheets of plastic when pre-exposing color transparency film. The object would be two-fold: Open up the shadows a touch, and reduce the bluish cast of shadows caused by illumination from the sky.
What # color compensating filter(s) are we going to use for this, and can anyone be specific about the type, thickness, and opacity of the plexiglass he mentions?