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 glass 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.
Please note that Singh-Ray thin mount filters do not have a front thread.
Additionally - the design of the Vari-N-Duo Filter may introduce irregularities 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.