Do you even know what that is? Do you know what a regular ND filter is? Let’s see if we can explain.
ND stands for Neutral Density. And it acts like a light restricting filter for your lens. By limiting the amount of light that comes into the lens, you can intentionally slow the shutter down in brighter light conditions that you usually would not be able to. So you can take flowing pictures of waterfalls in brighter conditions.
A variable ND filter is almost like you stacked several ND filters in place. It is fully adjustable. You can remove anywhere from 2 to 8 stops of light just by rotating the filter. The more you turn, the more light is removed.
With this technique, you can make a busy city street or highway, turn into an empty urban scene like what you see with the photos from photographer Matt Logue below. He made the street of LA utterly devoid of human life.
So what does one of these things look like? Like this:
It seems pretty standard. Now mounting the thing to your lens can be a challenge too. The above filter is 77mm. Or the same diameter as a 70-200 F/2.8 lens. So what do you do if your lens is smaller? You use a set of step-up rings. Like these:
Make sense? And how much does all of that cost? Not too bad. The filters you see above are good. If you don’t get a good one, you end up with bright spots in your picture and light leak issues.