This is brilliant. The Singh-Ray 77mm (Thin Mount) Vari-ND Variable Neutral Density Filter can adjust from 2 stops to 8 stops of ND.
You simply rotate the ring to either increase or decrease the ND effect. Its perfect for times when you aren’t sure how bright the light may be and provides an alternative to carrying a set of several ND filters. Snow, white beaches, waterfalls, any time when you may want a longer exposure.