Variable ND filters for Mattebox?

[Aldous Acuesta]

Hello,

I was doing some research on variable nd filters and wanted one for my mattebox then I came across the Fader ND Digi Pro-HD. It was only in 4x4 and I need one for my Cavision 3x3, so obviously it would not fit.

There are a whole bunch of DIY variable ND filters, but for the screw on lens filters only, that either use one linear and one circular polarizing filter OR two circular polarizing filters combined. When I looked at the video from Lightcraft Workshop for the Fader ND Digi Pro-HD, it showed TWO pieces of glass filters. One rotated while one stayed still.

So when I watched the video, I was thinking to myself: Is that just two polarizing filters as shown on the DIY threads? I didnt' want to go and buy another polarizing filter without asking here first.

- Should I go with one linear and one circular?
or
- Should I go with two circular polarizing filters?

Here is a link to video that I watched:
Filter > Fader ND Digi Pro-HD - Fader ND Digi Pro-HD ----4x4 inch (4mm thick) -

You can also google "DIY variable nd filters" for a whole bunch of tutorials. Sorry I do not understand the science behind all this, so please help me out. I currently have one Schneider Circular True-Polarizing Filter (3 x 3"). Thanks!