The one filter every photographer should have available for every lens they plan to shoot outdoors. The wavelengths of light in the world are oriented in any direction. When light reflects off of a non-metallic object like water or glass the rays become polarized: They are all oriented the same way and at the same angle to the viewer, resulting in glare. The sky also polarizes some of the sunlight as it passes through, resulting in some glare and reducing the blueness of the sky on bright, sunny days.
A polarizing filter only lets light through in a single orientation (by rotating the front of a polarizing filter you change what orientation of light you choose to let through). This lets you block all of the reflected glare from water, glass, shiny plant leaves, etc. The result of using a polarizing filter are bluer skies, elimination of glare ‘whiteout’, and usually an ability to see into water or through windows rather than simply seeing the glare reflected from them.
Unlike our high-grade Heliopan polarizing filters, these filters are generic mid-grade quality.