Two polarized glass panes whose rotation continuously adjusts light transmission — no color shift, no filter swap. Quick on-set adjustment for changing light.
Two polarized glass panes, the rotation of which continuously changes light transmission — without color cast, without new filters. Quick deployment in front of the lens in variable light. On set, you need this part when the sun moves but your ND set is at home or the transition speed between two fixed densities would be too abrupt.
How it Works and Practical Use: The mechanism operates via two polarizing filters — one fixed in front of the lens, one rotatable in front of or behind it. Depending on the rotation angle, you let more or less light through. At 0 degrees, maximum transmission; at 90 degrees, maximum blockage. The crucial point: You don't need color correction. Neutral density remains neutral. On set, I simply adjust the element as the sun rises higher — no filter changes, no cuts to swap. The image fluctuation remains smooth.
However, the system has limitations that you must respect. The maximum reduction typically lies around 2 to 3 stops — significantly less than an ND3.0 or ND4.0 provides. Polarizing filters can lead to moiré patterns or slight color distortions at extreme rotation angles with certain camera-sensor combinations. Always test the setup with your current camera, especially with high-resolution chips. And don't forget: when shooting polarized light in front of glass, reflections can react unpredictably — this is both a feature and a bug.
When to Use, When Not to Use: Variable density excels in outdoor documentary work, live events, and situations where you lack time for filter changes. For controlled studio setups or for shots with extreme ND requirements (more than 3 stops), I would opt for fixed ND filters or electronic ND solutions. The psychological component: the feeling of continuous adjustment looks cleaner on the monitor than repeated cuts for filter changes.