When it comes to enhancing images, a circular polariser is probably the oldest trick in the book. It can change a pale sky to a deep blue one, turn foliage into a luminous super-green, light up autumn colours, saturate rainbows and even make water look crystal clear. The added contrast, saturation and clarity that a polariser can give to images is nothing short of magic and every photographer should have one in their bag.
Contrary to it's name, a polarising filter does not polarise light; it filters out polarised light. Light from the sun is unpolarised and is then polarised when it bounces off any non-metallic reflective surface like water, foliage or various air particulates. Polarised light robs images of colour, clarity and contrast and is in many situations undesirable. A polarising filter removes this 'damaged' light and ensures that only unpolarised light reaches the camera sensor to produce a sharp, saturated and contrast-rich image.
For this reason, a polarising filter is a very popular tool for any photographer, but especially for landscape photographers.
NiSi's HD polariser is made from precise H-K9L optical proglass, which ensures optimal light transmission.
What is the difference between a linear and circular polariser? Digital cameras' AF systems are not compatible with linear polarisers. When the world was transitioning from film to digital, linear vs. circular was a big debate, but it no longer is. For photographic purposes, you can assume that any polariser on the market nowadays is circular and not linear.