2 polarisers?

[Squidy]

So, as Lee filters were too expensive I bought a NISI filter system a few months ago. See https://nisifilters.com.au/category/nisi-100mm-square-filter-system/

Now, the filter holder has a circular polariser built in (that you can unscrew and remove, leaving just the holder). They also see a more high polariser as seen here https://nisifilters.com.au/product/nisi-100x100mm-square-hd-polariser-filter/

My question is, if I've got the more expensive polariser am I meant to leave the circular one in? I'm pretty sure this will cause vignetting? If not is it too much? Should I remove the circular one and just use the HD one?

Sorry for the question, I just want to make sure I'm doing this right and getting the correct results.

[NFLD Stephen]

Not familiar with this product specifically, but I believe the intention is to only use one of the polarizing filters. But if you were to use two, you can vary the amount of darkening by rotating one of the filters in relation to the other one. This is how a variable ND filter works. Just try it out: hold two polarizing filters up to your eyes and rotate them both. You'll see pretty quick the variable darkening effect.

Stephen

[Jonathan Huyer]

I agree with Stephen, if you leave the two polarizers in then you will have a homemade variable ND filter. If you just want the polarizing effect, then unscrew the base filter and go with your nicer one. When using the separate polarizer, there should not be a difference with vignetting with the inner filter installed or removed. But it is possible that you will have less vignetting if you use the built-in polarizer alone, and not the separate polarizer, because the separate one sits a little bit farther away from the lens.