SAVE $80 Today on Tiffen 77mm Variable Neutral Density Filter (Price After Rebates: $69.95). Variable neutral density Filters are handy for controlling exposure for both stills and video because they allow you vary the amount of neutral density from two to eight stops by rotating the filter.
The heavier the density, the wider your aperture or slower your shutter speed can be. This comes in handy for extreme time exposures to create movement within the shot. Variable ND filters eliminate the need to buy and carry around individual neutral density filters. With this one filter, you can rotate to the degree of ND you want – a real time and money saver.
Since variable ND filters aren’t cheap, I advise buying one in the largest filter size you need and using inexpensive step-up adapter rings on your smaller lenses. Using a step-up ring to an over-sized filter also helps prevent vignetting when shooting with extremely wide-angle lenses