Variable ND filter

Two polarising layers rotate against each other and the density rises as they approach cross-polarisation. That saves swapping fixed filters every time a cloud passes, which is why they end up on run-and-gun and outdoor rigs.

They have a limit worth knowing. Push towards maximum density and you get the X pattern, a dark cross across wide shots, along with colour shifts and softening, and cheap ones start misbehaving well before the end of their range. The usual advice is to buy for the stops you actually need rather than the widest range on the shelf, stay off the last part of the dial, and check the corners of a wide frame before you roll.