I think the 82mm filter size of the 16-35mm MkII is irrelevant because in general, I really recommendagainstround GNDs. The gradation, whether hard or soft, is in the middle of the circle - exactly where you usuallydon'twant to place the horizon line.


So, my recommendation is the same as for any other lens - a rectangular filter (Cokin for the budget-minded, Lee or Singh-Ray if you want better optical quality). Both Cokin and Lee make 82mm adapters for their filter holders, or you can simply hold it by hand.