What is an ND filter?
ND (Neutral Density) filters are unique tools in photography, not only landscape. It is not unusual to see beautiful photos with the presence of silky water, sea without ripples, or streaked clouds and very soft light. Obviously with the advances in post-processing, many will think about the use of retouching software, believing that anything can be “fixed in post”. However, as most professional photographers will tell you, the best results are achieved in camera with ND filters.
So what is an “ND” filter? The abbreviation stands for “Neutral Density”, literally they are colour neutral. Filter manufacturers produce ND filters in glass (of varying degrees of quality dependant on the manufacturer) or resin (a lower quality optical product) and are specifically designed to evenly reduce the transmission of light across the spectrum.
The more advanced the quality of the neutral density treatment, the more evenly the colours of the original scene are transmitted to the sensor. The uneven light transmission will allow more or less colour through, in relation to the others, resulting in that colour being overly present. Commonly known as colour cast or the presence of “a dominant” colour. if you have a low-quality filter giving a strong cast (most commonly blue or magenta) you can sometimes use White Balance to compensate – but we think it’s better just to produce a truly neutral density filter in the first place. Also, in many cases, the differences in colour are not even in lights and shadow, or too much to compensate in post-production, ruining the photos. Read how the best ND filters are made here.
ND filters are an additional method (after ISO, aperture and shutter speed) to manage the light when shooting; a critical tool in the hands of the photographer to be able to get the results he wants.
What ND filters are for?
An ND filter allows you to lengthen the shutter speed. The light reduction of a filter is measured in absorbed “stops“. At each stop the shutter speed must be doubled (assuming f/ value and ISO stay the same) to obtain a photo with exposure similar to that of the unfiltered scene. For example: without a filter we take a well-exposed photo with half-second shutter speed, but we want to get silky water and accentuate the movement of the clouds. We introduce a 6-stop ND filter and the light is reduced, therefore the shutter speed can be slower, by how much?
This is because we must double the shutter speed for each stop of light absorbed. In this case for 6 times, i.e increase by 2 to the power of 6: 2 x 2 x 2 x 2 x 2 x 2 = 64.
The ND filters are also marked with this value, you can see 6-stop ND filters indicated as ND64. A half-second without the filter corresponds to 32 seconds with an ND64 filter. Taking a photo for 32 seconds with the filter will result in silky water and some hints of movement on the clouds, nothing prevents you from using much darker ND filters and trying longer times. NiSi offers filters up to 20 Stops.
Types of ND Filter
We pride ourselves on having a comprehensive range of products to suit all photographers needs: square and circular screw-in in different sizes and densities. We offer products of the highest quality to interfere with the quality of your lenses, take a look at some of the options.
Circular screw-in filters
These are the most common ND filters, they are screwed directly on the lens front thread. Being only as large as the diameter of the lens they often do not exceed 77-82mm, therefore they cost less (less glass = less cost). They are perfect for getting started but fail to have the same versatility as square filters, which we will see now.
Square ND filters
Square Neutral Density filters require a filter holder such as the NiSi V6, and must not be screwed directly to the lens. This offers undeniable advantages and versatility. With the NiSi V6 holder you can add up to 3 square filters (ND and GND, for example) and also mount the included polariser. Now let’s see the two main materials used by manufacturers for the construction of ND filters.
Resin Neutral Density filters
The cheapest ND filters are built with optical resin (similar in appearance and texture, although chemically different, to plastic.) This resin is dunked in tanks of heated dye to give the ND effect. This method of production is now obsolete. Resin, regardless of quality, is optically inferior to glass. Learn more about the importance of optical glass in photography.
Glass Neutral Density filters
Optical glass is the logical choice to make ND filters, after all, it is the same material which photographic lenses are made. Most manufacturers use glass and apply a layer of dark coating (even by hand) to match the right gradation. The method used by NiSi is much more precise, automated, computer-controlled and measurably superior. NiSi uses only an exclusive lens grade optical glass of the highest quality. NiSi applies his ND filters a series of patented treatments, such as Nano Coating®, IR for blocking infrared light and anti-reflection treatment.
What ND filter to choose and why
Choosing an ND filter is simpler than you might imagine; the first question to ask yourself is “screw-in or square?” If you have never used an ND filter perhaps it is wiser to start with a circular ND filter, the expense will be lower and you can get used to using ND before moving on to the much more practical square filters.
If you already have some experience, or if you want to use GND filters, the answer is square filters.
If you choose circulars you only have to look at the size of the thread of your lens, often written directly on the barrel. If you use a wide-angle it will probably be between 72mm and 82mm. Just choose a filter of that diameter and go out and shoot.
For square filters, however, you will not have to choose a filter for each size, but go directly to the V6 holder that allows you to mount 100mm wide ND square filters with 67-72-77-82mm lenses, all included in the initial kit (including polariser).
If other dimensions are needed, just add a step ring (available up to 49mm here). If you have a lens that doesn’t support screw filters, e.g. ultra wide-angle lenses with a protruding front element, you can use a 150mm NiSi holder for your lens such as the classic 150mm holder or the NiSi S5 (similar to the V6, with polariser included). In this case, using 150mm wide filters.
Read our guide to NiSi filters Systems
Colour cast in ND filters
Colour cast in ND filters is the single most important thing to consider when buying these products. Nobody wants to finally get the shot and then see it ruined by a blue or magenta colour cast. For this reason, it is necessary to thoroughly check the type of coating and glass used.
Many of these treatments were introduced by NiSi, and we are the only manufacturer in the world to use these processes together. In addition to the highest quality lens grade glass on the market, NiSi ND filters also have a special anti-reflection coating that minimises reflections that could cause flare. For further control on the final result, a coating for the absorption of infrared light (IR) has been added, in this way the dominant colours are avoided on longer exposures where precision is fundamental.
The photos speak for itself, below you can see the difference between a NiSi ND filter and a counterpart. At the top a reference photo without any filter. Find out what the author of these shots, Francesco Gola, thinks in his review of NiSi filters.
NiSi ND Filter Vs. Other ND Filter