Polarizer

Kenko CIR-PL Celeste 82mm 300i

1 day:	300i
from 3 days:	270i
from 14 days:	225i
from 30 days:	210i
from 6 days:	210i

Marumi CIR-PL 52mm 300i

1 day:	300i
from 3 days:	270i
from 14 days:	225i
from 30 days:	210i
from 6 days:	210i

Gizmo CIR-PL 58mm 300i

1 day:	300i
from 3 days:	270i
from 14 days:	225i
from 30 days:	210i
from 6 days:	210i

Hoya CIR-PL 46mm polarization 300i

1 day:	300i
from 3 days:	270i
from 14 days:	225i
from 30 days:	210i
from 6 days:	210i

What is it and why is it necessary?

Here are polarizing filters

A polarizing filter or polarising filter is often placed in front of the camera lens in photography in order to darken skies, manage reflections, or suppress glare from the surface of lakes or the sea. Since reflections (and sky-light) tend to be at least partially linearly-polarized, a linear polarizer can be used to change the balance of the light in the photograph.