A Vari-ND filter is a filter made of two circular polarisers stacked one on top of the other. I bought the K&F Concept varied-nd which is advertised as and ND2-ND32 filter. I bought the 52mm diameter so it fits on my 35mm f/1.4 and 18mm f/2 lenses that I often use in street photography.
Light is an electromagnetic field which is emitted and travels with every angle from 0° to 360°. A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed polarization into a beam of well-defined polarization, that is polarized light. (from wikipedia).
When a second polariser is stacked on top and in the same direction, it has little to no effect. However if the second polariser is rotated then it will cut out the light coming from the first polariser. The more it is rotated, the more light it cuts out
I can now control with a twist of the filter the amount of light that I let into the lens. I hear that these filters are not of a good enough quality for long exposure landscape photography. The light is too uneven across the frame and these filters do not give a good enough sharpness.
My idea is to put my camera in it’s lowest ISO setting (160 for the Fujifilm x-pro3). I choose quite a small aperture (from f/5.6 to f/11) and I twist the the filter until the shutter speed slows down around 1/6s or 1/3s. This will give me some blur in the photo.