Physics in action

On the right is a circular polarising filter and to the left is a linear polarising filter. Place the linear polariser in front of the circular polariser[1] and they will block between none and all of the light as you rotate one of them.

Tada! You've now got yourself a variable neutral density filter.

Now I need to pull the finger out and use the bugger. I'll probably take a wee trip to the beach if the weather isn't too disgusting at the weekend.

[1] I think that this would work with two linear polarisers and that the circular polariser is needed for the autofocus system.