Cokin (P Type) Circular Polarisers

[hangerhead]

I already got a (KOOD) CPL for the Cokin P system - it's a simple disc that slots into a disc holder which i can use on the cokin filter holder.
however, the CPL is only a single sheet of glass - i though that proper CPLs required 2 pieces of polarised glass, one of which rotates.

On ebay for example are Cokin P164 filters, again made in the same arrangement.

Am i concerned over nothing?

p.s. I shoot digital only - perhaps this cokin filter is really a cicular style BUT LINEAR ONLY polariser - these i think work with film but not digital.

[Marcel]

Circular polarisers are one sheet of material too.
They are made in such a way that at certain angles the polarised light goes through, and at others, the polarised light bounces off.

[hangerhead]

ohhhhhhhhhh.
ok.

it's just because on a previous systen I was using, there were two pieces of glass, one of which rotated, etc.

Thanks for the info.