Variable ND filters
Variable NDs are very convenient when it comes to controlling the amount of light entering the lens. They are always the screw-in type and can range in density from as much as 1 to 8 stops, essentially offering the flexibility of eight individual filters. So how do they work?
Once attached to your lens a variable ND is rotated until the desired density is reached using a scale on the side of the filter. They’re made of two sheets of polarising glass that darken as they are rotated. These filters are great if you’re on a budget or want to keep kit to a minimum, but this also comes at a small cost. Many variable NDs suffer from a large X that appears in the frame when close to, or at, their highest strength, but this is easy to avoid.
Exposure 2mins with 10 stop ND
Exposure 1sec with 3 stop ND