I knew variable ND filters were immensely useful since you could continuously shoot at the optimum aperture without the need of changing the filters according to the situations. I, however, had never used any variable ND filters that satisfied me in terms of image quality. Variable ND filters I knew could not reduce the light entering into the lens evenly, resulting in poor image quality.
This unevenness is called the X effect as the darker part creates an X-like shape. NiSi claimed that they completely eliminated the X effect with their new ND filter, ND-VARIO Pro Nano. So I decided to give it a try. As usual, I chose the bigger one, 82mm, so that I could use it with most of my glasses. The available filter thread sizes are 67mm, 72mm, 77mm, 82mm and 95mm.