The CP is actually a sandwich of a LP with a quarter wave plate on the camera side, this removes the linear polarization of the light after it has already been filtered, and allows the cameras autofocus to work.
So, by adding the LP to the outside, what you actually have is LP/LP/quarter wave plate/lens.
The the nice thing about the variable ND, is you can still autofocus, then rotate the outer LP, to darken it.
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