Circular Hybrid Polarizer Filter
Moose Warm Polarizer filters, an exclusive collaboration between Hoya and respected wildlife photographer and conservationist B. Moose Peterson of Wildlife Research Photography, are a combination of a polarizer and an 81A warming filter.
For many years, polarizers have been used to remove reflections from non-metallic surfaces such as water and glass as well as being used to darken blue skies to increase contrast in scenic photography.
When using a camera with a split beam metering system (a metering system that employs a polarized half mirror) which is most of today's auto focus camera bodies, traditional linear polarizers will cause exposure errors due to their light absorption properties. Circular polarizers yield the same optical effect while not causing exposure problems with modern metering systems
A side effect of both linear and circular polarizes is they "cool down" or make the over-all color balance of a scene slightly bluish. The addition of the 81A glass corrects the color temperature, bringing the scene back to the original 5500K for daylight film's color balance. The 81A glass creates a much more pleasing and warm color balance to the entire scene while the circular polarizer increases color contrast and reduces the effect of atmospheric haze.