Short Answer: This is something we generally recommend against, because VNDs and polarizers will interfere with each other. The end result in most cases is cross-polarization.
Quick Summary
The reason cross-polarization occurs is because VNDs use polarization physics to create the variable-density effect. A VND essentially consists of two polarizers, facing in opposite directions. One remains fixed while the other rotates, and the way the two polarizers interact is what causes the change in the stop value.
Longer Version:
Circular polarizers contain a quarter-wave plate in the glass. This element is what gives them their ability to cut glare and reflections. There can be a maximum of two quarter-wave plates in any given filter. A popular analogy for how polarizers (via the quarter-wave plate) work can be found here:
Picket Fence Analogy
VND filters essentially consist of two polarizers, and if their "picket fences" (polarization angles) align in the correct way, they can work together to cut exposure. This means all VNDs inherently contain two quarter-wave plates. At the same time, rotating the two polarizers too far can cause cross-polarization. You can simulate a crude VND by holding up two CP (circular polarizer) filters, and rotating one while holding the other still. The trick is to have them facing in opposite directions, since polarizers only work one way (and not rotating either one too far). Aligning them for specific stop values is tough to do custom, but that's where our design team comes in. We calculated the optimal stop ranges to avoid cross-polarization, and we added hard stops to prevent the filters from being turned too far in either direction. That means you can be sure our VNDs (or VND/PL hybrid filters) won't cross-polarize.
Common FAQs
Does that mean any VND is already going to work as a polarizer?
Not for practical purposes. Since there are two quarter-wave plates involved, and the hard stops on either end limit the range of rotation, a traditional VND is not going to behave tshe same way a regular polarizer does.
Is there any way to have polarization while also still using a VND?
Yes. A dedicated Chroma VND/PL filter will let you do this. The polarizer in those filters is set up to operate as part of the VND, which eliminates the interference you'd normally see from stacking the two filter types.
Does any of this apply to the Shortstache filters?
Yes, it does. Both the Everyday and the Mist-PL filter are polarizers, so we do not recommend stacking them with separate VNDs. A Chroma VND/PL Black Mist filter would be recommended instead.
Questions, comments, or concerns? Send a quick email to support@polarpro.com and we'll be happy to help out.