04-06-2018 05:14 PM
You're exactly correct. It's two polarizers rotating on a common axis. That also causes potential problems with a mark that resembles the letter "X" at some settings.
But for me, while experimenting, I'm willing to live with that given the convenience and it's fun to play with. For people who do professional or precise work, I think they need to get a regular ND filter.
I'd expect that "X" effect to be most pronounced in landscapes taken with a wide-angle lens, because the degree of polarization of sunlight depends rather strongly on the position of the sun relative to any given point in the scene. If only one polarizer is used, the usual effect is a gradation in the darkness of the sky. With two polarizers, it's no surprise if it's geometrically messier.
04-06-2018 06:28 PM
Thanks for the note. I haven't seen it yet in my photos, but an example on the web on the Petapixel web site is a landscape, just like you said. The site also says the effect is most pronounced at the darkest settings. Some people attempt to fix it in digital post processing.
One brand claims to have found a way to avoid that, but I don't have personal experience.