Singh-Ray Variable Neutral Density Filter - 77mm
This adjustable neutral density filter lets you adjust the Neutral Density affect by simply spinning the end of the filter, similar to how a circular polarizer operates. It lets you control up to 8 stops of light, allowing you to shoot at wide apertures in bright sunlight as well as other applicatinos. It will fit all lenses with a 77mm filter diameter, but there is also a 77mm thin-frame version that is intended for super-wide angle lenses which may otherwise vignette with the thicker frame version.
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