The Singh-Ray 77mm Vari-N-Duo Polarizing Variable Neutral Density Filter gives you the incredible benefits of two filters in one. With this unique filter, you can not only slow down the action but add in the effects of a warming polarizer.
Heretofore, when photographers wanted to create this effect, they had to "stack" filters on the lens - a sure ticket to vignetting. Now, this all-in-one system permits you to carry one filter to achieve the desired results. You can easily adjust the neutral density from 2-2/3 to 8 f/stops and, at the same time, control the density of your sky, make clouds stand out or eliminate reflections. The Vari-N-Duo allows you to not only create some stunning blurred images but with various degrees of polarization as well.
There are two rings on the filter - Min/Max for controlling neutral density, and a rotating polarizing ring just behind the Min/Max Ring.
The design of the Vari-N-Duo Filter may introduce irregularities or vignetting when used with very wide angle lenses, especially on full-frame cameras. Adjust your focal length and reduce the filter density setting until the irregularities disappear
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Singh-Ray 77mm Vari-N-Duo Polarizing Variable Neutral Density Filter (Standard Mount)