[kypfer]

Also beware variable ND filters with wide angle lenses. These things are 2 polarizers and can create a darkened cross with WA lenses.

In my experience, the "dark cross" problem only occurs when both elements in the filter are circular polarisers.
With a circular polariser fitted to the lens first, then a linear polariser fitted on top, no "dark cross", similarly if both elements are linear
If you fit the linear polariser first, then the circular polariser, the whole concept doesn't seem to work … didn't bother to figure out why!
Obviously, with two rotating filters, getting the adjustment "just right" can be fiddly. I've gone to the extent of gluing one polariser so it won't rotate … if I just need a polariser I can still use the other one, but a scrap of adhesive tape would probably work just as well and not be quite so over-kill

[Sakura]

This is minimally processed and the original crop.

Thx for sharing, never seen the polarizing effects with short lenses before. But it is good visible in you're picture.
I think you did a good fix to minimize the effect (although it is still viable if you know that it is there)

[Digitalis]

In my experience, the "dark cross" problem only occurs when both elements in the filter are circular polarisers.

can confirm. I own two linear polarizes from Lee and they both go quite dark when set at the right angle, but if I use two circular polarizes...you get the cross effect, also a slight blue cast which is undesirable.

[Sakura]

can confirm. I own two linear polarizes from Lee and they both go quite dark when set at the right angle, but if I use two circular polarizes...you get the cross effect, also a slight blue cast which is undesirable.

Just checked my low-cost variable ND filters from aliexpress, they are liniear filters.

Presumably this is correct:

To distinguish a Circular Polarizer from a Linear Polarizer, turn the filter backwards and look through it into a mirror. If the filter image in the mirror is black, you have a circular polarizer. If the image is clear, you have a linear polarizer.

So i really wonder, could the coating between cheap and expensive ones be the only difference?

this are the filters i bought, and they work like a charm.
FOTGA Ultra Slim 40.5 82mm Fader Adjustable Variable ND Lens Filter ND2 ND8 ND400 Red|Camera Filters| - AliExpress

[Digitalis]

So i really wonder, could the coating between cheap and expensive ones be the only difference?

There is more to a circular polarizing filter than just the polarzing film, there is also the quarter wave plate - which is the component Linear polarizers lack.

[Sakura]

I know circular polarizing filters are preferred for single polarizing filters.
But i understand that for variable ND filters you need linear and not circular. My question was more what the difference is between low cost variable ND filters and the more expensive ones.

[Digitalis]

The expensive Variable ND filters are less likely to have residual colour casts and will reduce exposure more than the cheaper less efficient ones.