Neutral density filters come in a variety of densities, graduations, shapes and sizes. Generally speaking, ND filters are single plates of glass (either square or circular) that are designed to reduce the light that is available to a camera sensor by a known, set amount – e.g. but not limited to 3 stops (ND8), 6 stops (ND64), 10 stops (ND1000) or 15 stops (ND32000). By reducing the available light, you can control exposures in bright light, get creative with longer shutter speeds to capture motion, or control apertures, shutter speeds and frame rates when producing video.
Variable neutral density filters are different in their design in that they allow you to vary the degree of density by rotating the front element of the circular filter. Rather than being a single plate of glass, variable ND filters are effectively two individual polarising glass elements, the rear being in a fixed position, and the rotating front element fixed in opposition to the rear. As the front element is rotated, and the nearer to alignment that the two elements come to each other, less light is being allowed to enter the camera. By rotating the filter throughout its 360-degree range, you are changing the density of light reduction from its lightest to darkest through a smooth transition, with the NiSI VARIO Pro Nano range of variable ND filters offering variations from 1.5 – 5 stops of light in circular filter thread sizes of 40.5mm to 95mm.
Variable ND filters are popular within the video production market and allow you to control your aperture, shutter speed and frame rates quickly and easily through rotation of the filter, reducing the need to swap fixed circular neutral density filters in and out. The smooth transition and high build quality of our NiSi VARIO Pro Nano variable ND filters can help eliminate vibration or camera shake when making density changes, and the high-quality optical glass and design of the different front and rear glass elements ensures there is no noticeable “x-effect” (sometimes referred to as the Maltese Cross in photography) when the filter is at its lightest or darkest setting. No noticeable color cast or uneven color banding will be noticeable throughout the entire range of density from 1.5 – 5 stops in our variable neutral density filter range.
For still photographers the convenience of having a single circular variable ND filter could save much-needed weight and space in your camera kit if all you require is a filter with a maximum density of 5 stops. For those that require filters darker than 5 stops, we have a full range of neutral density filters in fixed variations in our circular neutral density filter range. If you need the ability to stack filters, square neutral density filters in either the 75mm, 100mm, 150mm or 180mm NiSi product ranges may be the better option.
Advantages of using variable neutral density filters
Single filter for multiple densities – Having a single filter that can be rotated to vary the amount of light can be very convenient and save space in your camera bag.
Potential cost savings by purchasing a single filter – High-quality variable ND filters tend to be more expensive than fixed density filters of the same size. If you calculate the cost of purchasing a range of filters in different densities available on a single variable ND filter, the cost can add up quite quickly. A single filter may save you money if you intend to use it long term.
Reduced vignetting when using a single circular filter – stacking circular neutral density filters together is not recommended due to vignetting. By using a single variable neutral density filter you eliminate the need to use multiple filters to achieve the same maximum density.
Good for video – By having a quality variable ND, you can reduce the need to swap out ND filters when needing to control aperture, shutter speed and frame rate while shooting video. A high-quality variable ND filter will have a smooth rotation while changing densities, which can help to decrease and vibration introduced through the lens.
Disadvantages of using variable neutral density filters
Image quality issues and “x-effect” with cheaper filters – Not all neutral density filters are created equal. With many cheaper variable ND filters on the market you tend to find issues caused through lower quality glass or resin, and also will quite often notice the “x-effect” or “Maltese cross” caused by filters with too much of a variation in density range (eg 2 – 10 stops). Due to the way the different elements of variable ND filters line up to produce the desired density, banding and uneven exposure throughout different areas of an image can also be noticed with many cheaper filters on the market.
Cannot stack graduated ND filters – Circular ND filters quite often create a vignette effect when being stacked with other filters. It is not recommended to use graduated ND filters in either round or square variations in combination with variable neutral density filters.
One size does not fit all lenses – if you have a variety of lenses, circular ND filters will need to be purchased for each lens. Due to the way variable ND filters alter the density of light through the variation of alignment of the different glass elements and allow it to be transmitted through to the camera sensor, using adaptors to step down variable ND filters is not recommended. You are best using high-quality variable ND filters that match the size of your specific lens threads for the best result.