Polarising filter

A circular polariser blocks light waves oscillating in one direction. Turn the front ring and reflections on glass, water, paint and foliage fade in and out, an effect no amount of grading recreates afterwards. It costs you one to two stops of light.

Wide lenses bring two catches. The sky darkens unevenly across a wide frame, so a 16mm shot can finish with one patch of navy and one of pale blue, and panning during a take shifts the effect mid-shot. Set it for the shot you are on and check it again after every reframe.