All photography and video enthusiast will sooner or later come across a variable ND filter. Variable Neutral Density filters have pro’s and cons, but the cons are mostly restricted to low-cost products which can completely ruin a photo or video.
The relatively low cost of a Variable ND’s and the possibility of having “several ND filters in one” by varying the absorption of light, has made this product very popular among those who want to start shooting long exposure photography, or simply want to manage the light more effectively in photo and video.
We will see how the quality of the Variable ND is of crucial importance for the photo or video you’re shooting. For further information on fixed ND filters you can refer to this article.
How a variable ND filter works
A variable ND filter is made up of two polarising filters set in opposition to each other. By rotating one of these we obtain a decrease in the light that passes between the two polarisers and different manufacturers block the rotation up to the absorption value (stop) they intend to achieve. Furthermore, it is not possible to have a variable ND filter that starts from zero, all filters have a minimum absorption of 1 or 2 stops.
How to use a variable ND
The Variable ND filter is used in a similar way to classic ND filters, but there are some useful advantages due to the possibility of varying the strength. With a normal ND filter, especially from 6 stops up, it is preferable to compose the photo or video and then attach the filter afterwards. With a variable ND filter, however, this is not necessary as we can rotate the ND to vary the absorption enabling you to set your composition with the filter in place. The fundamental parameter to consider is the shutter speed that we want to obtain. The neutral density variable filters are always screw-on and usually available in the most classic sizes of 67, 72, 77, 82 and sometimes even 95mm. Read our step by step guide to long exposure.
Example of effective use of a Variable ND
For example, we want to capture the movement of the waves but without making the sea too flat. We will probably look for a shutter speed not higher than 1″ or in any case around this value. Once the shutter speed has been decided, we set the aperture which allows us to obtain the maximum optical quality and depth of field. At this moment you probably don’t have the perfect exposure (unless luck is on your side), so the ND comes into the equation. Rotate the variable ND to the desired exposure. Convenient right? But it’s not all perfect, there are several factors to keep in consideration for the choice of a variable ND filter. We will come back to this.
Example of use of a Variable ND for video
The use of variable ND in cinematography is even more important than in photography. Sometimes it is an indispensable tool if we want to affect the exposure during shooting without changing the depth of field, “running and gunning”. During video shooting the shutter speed is very important and must be set in order to obtain the right fluidity, it is therefore not a value that we can change at will. The same goes for the aperture, especially in some scenes or particular shots where we want to keep full control of the depth of field. For this the variable ND allows you to adjust the amount of light that reaches the sensor and obtain the perfect exposure even with changes in framing and light. You can find a review with video tests here.
Main characteristics to consider in a variable ND
The main features to consider in a variable ND filter are the structure (frame) and design, the quality of the glass, the polarising film, the X effect (or cross effect), the external treatments and finally the fluidity and ease of rotation.
External structure
You probably don’t care a great deal about the material or external design of the variable ND filter you’re buying, yet it can bring many advantages (or disadvantages). The material must be robust with particular attention to expansion and shrinkage due to temperature variance, the design must be as thin as possible to avoid light falling at the edges (vignetting). Furthermore, a variable filter very often introduces flares and chromatic aberrations more easily than other filters.
Glass quality
The quality of optical glass is certainly the aspect that immediately comes to mind as soon as we talk about a filter for photography. Be wary of filters that do not clearly state that they use optical glass, and even in this case the differences are many. Obviously everything that is resin, ordinary or unspecified glass is not even worth considering. Any photographic lens, even of compact cameras, uses lens grade optical glass in the construction of optical schemes. And so we use lens grade optical glass in the manufacture of all our filters – it simply wouldn’t make sense to use anything else. Learn more about the quality of glass in photographic filters.
Polarising Film
The polarising film, in this case the two polarising films, affect both the precision of the Variable ND filter (do the stops indicated by the manufacturer correspond to reality?) and the chromatic fidelity of the photo/video. Unfortunately, very often manufacturers do not indicate which polarising film they use, often because it is not a quality film, it is not a recognised brand or they simply do not want to reveal it. An excellent brand, a leader in the market, is Nitto. You can find out more about polarising filters here.
Filters coatings
The surface treatments (coating) that can be applied to filters for photography are manyfold; among the most innovative there is NiSi nano-coating, which makes the surface of the glass water-repellent and protected from the natural oils of the skin, making filter cleaning fast and effective. On high-end products, we can also find anti-reflective treatments. Most variable ND filters, however, do not have any particular treatment, as production costs can rise even without using these expensive treatments.
Ease and fluidity of rotation
Another unique feature of the variable filters is the rotation, which does not occur in those with fixed neutral density. In addition, the vocation of variable ND filters is clearly that of videos, therefore ease and fluidity of rotation is a determining factor in order to vary the exposure even during shooting. Some filters also have a small grip to allow easy rotation, at the bottom of the article we will see two of these, the NiSi ND-VARIO Variable 1.5-5 stops Nano-coating Enhanced and 5-9 stops.
X effect and aberrations
The so-called X, or cross effect, is an effect that occurs with variable ND filters at certain absorption intensities, the photo or video is compromised when this effect occurs. Many manufacturers prefer to push their variable ND filters beyond 7 absorption stops, even reaching 9 stops or 10. This choice causes the formation of the cross effect on most of the various ND filters, only within about 5 Stops and maintaining a high quality you can be sure to avoid the X effect. In the image below you can see the classic cross effect in the photo on the left.
NiSi ND-VARIO Variable 1.5 – 5 stops Nano-coating Enhanced and 5 to 9 stops
NiSi has developed two variable ND filters taking into account all the technical characteristics necessary to obtain a product of the highest quality at a lower price than the top of the range.
- The structure of the NiSi ND Vario is an alloy highly resistant to temperature changes, as well as with a very low profile, minimising vignetting
- Lens grade optical glass , exclusive to NiSi which is famous for its extreme quality and colour fidelity.
- Nitto polarising film of the highest quality and colour matching.
- Nano-coating and anti-glare treatment
- Smooth and precise rotation, external screw for comfortable rotation
- Effect X absent in any position from 1-5 to 5 stops and from 5 to 9 stop*
- Thanks to the precise construction, both filters are free of strong cross-effect
- Find out more in our ND-Vario page
Anything you’d like to ask us?
*NiSi variable filters are the state of the art for a variable filter, but in any case, variable filters are not real ND filters (of any brand). A variable filter is always constructed using paired polarizers. NiSi uses the best quality glass and pays maximum attention to construction and polarizing film, plus it designs, manufactures and distributes its products directly. A variable ND filter will never have the same quality with the same construction and materials as an ND filter. Therefore, we always recommend a true ND filter for specific applications where the advantages of a variable filter are not fundamental.
