Before we can delve into what variable neutral density filters are and how they can make a significant impact to your photography and the weight of the camera bag let’s take a flashback on what neutral density (ND) filters are. Neutral density filters can be termed in a single phrase as an eyewear for your camera. When screwed onto the front part of the lens they immediately reduce the amount of light that comes through the lens. But wait a minute isn’t good photography about light and more light is better? Yes and no. Yes because good photography depends on light and no because excess amount of it has the effect of washing out your compositions. Say, when you are trying to capture a tide with a brightly illuminated sky in the background. You want to capture the tide moving in and want that dreamy effect where the water is all white and silky in appearance. This needs a long exposure. However the problem is with a long exposure, your camera sensor is going to capture so much light that the final image is going to be a white canvas. So what to do? This is where a neutral density filter comes in. They work just like sun glasses, stopping the excess amount of light and allow you to have a long exposure. They come in various f stops or amounts of light that they can stop.
So what is a variable neutral density filter? A neutral density filter is great except that it cannot be adjusted to variable densities. As a result a photographer who relies heavily on neutral density filters has to carry several of them designed to stop light in increasing stops (please note every stop here is a half of the previous, so that one stop means half and two stops means quarter and so on). This is why a variable neutral density filter is so much effective.
A variable neutral density filter sits in front of a camera lens just as a normal neutral density filter would. You will have to screw the filter on to the lens. Now turn it slightly in any direction and the amount of light stopped increases. As such it is like a sunglass for the eyes but that is conditioned with a turn screw. You turn it to adjust for the amount of sunlight that you wish to block. Sounds amazing? It is indeed so, it is almost like magic. Variable neutral density filters immediately creates a situation where there is less need to carry several different neutral density filters with different stops. Some people have tried using two ND filters to see if they can really cut out light for some very fast apertures. But that can cause some problem of vignetting and in that case if you are using full frame bodies you may have to zoom in a bit to avoid the corners. Having said that with a variable neutral density filter this problem will never arise and the photographer can simply turn the front end of the filter to adjust the amount of stop he needs.
Find out more on VND here Tiffen 82mm Variable ND filters review