Hoya 77mm Blue Intensifier Glass Filter
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Of "didymium" glass, this filter increases the color saturation of the blue spectum.
It is achieved through the selective blocking of the colors around the color being intensified.
Through this process Hoya has developed a filter that subtly increases saturation of blues and blue-greens such as sky and foliage while not giving an over-all color cast to the photograph.
The filter is most useful for photographing outdoors in noontime sun, when a circular polarizer is least effective for darkening blue sky.