It’s been about six years since I wrote on this topic and while the concepts have not changed, some of the vendors and methodologies have evolved. A recent question via email has prompted this new article.
What’s the Purpose
The goal of a neutral density filter is simple. It has only ONE job and that is to reduce the amount of light transmitted to the camera sensor. Neutral means that no colour shift can occur or should be tolerated. Density means that the reduction is consistent across the field of reduction.
Common use cases include reducing the light to allow for longer shutters speeds such as would be employed to allow for motion blur. Another popular use case it to use very large apertures in brighter light to provide for shallower depth of field. Videographers will often use NDs to adjust the light transmission to allow the camera to be set at a consistent gain level in differing lighting condition. Grads are used to manage dynamic range, often to bring down the brightness of a sky when shooting a landscape to allow for better shadow exposures without blowing out the brights completely.
Types of Neutral Density Filters
ND filters come in a variety of mounting types and constructions. The most common is the screw on ND. Some screw on filters are marketed as Variable NDs, most of which are utter crap. Variables will often provide between 2 and 6 stops of reduction. A Variable is two polarizers mounted together where one rotates and the the other is fixed. Then we have the square or rectangular types that slide into some form of filter holder. There exist some specialized ND filters including grads, tinted grads and spots, although spots are so rare now that I won’t spend time on them.
The Numbering, Or How to Confuse Buyers
ND filters are measured in the amount of light reduction, most commonly in stops. This is then translated into an allegedly easy to understand numeric qualifier, which would be true if every maker used the same scale, but they don’t. Thus a 3 stop ND filter can be simultaneously referred to as a 3 stop, an ND8 or an ND 0.9 and all names describe the same amount of light reduction. Getting from a 3 stop to an ND8 means taking the stop count and raising 2 to that power, in this case 2 to the power of 3 or 8. Getting to a 0.9 requires recognition that 0.3 is the same as 1 stop in the different measuring systems.
Selecting an ND Filter
Consider that most everyone has a polarizing filter which delivers about a 2 stop cut in the light transmitted, thus there is no reason to buy an ND of less than 3 stops. The most common ND used varies based on information source to be either a 5 or 6 stop ND. Because an ND filter cuts light before the light meter and the autofocus sensor, you should expect that for higher density ND filters that your light meter may provide poor readings as they only work to a certain light level and that autofocus is likely to be out of luck for the same reason. Consequently, if you are looking for ND filters offering 5 or more stops of cut, the current best practice is to go with ND filters of the slide in type.
Screw In Filters of Fixed Density
When looking at a screw in filter, always buy the right size for your lens or a larger filter with a step down ring. Never use a smaller diameter filter or you will get vignetting. Only choose ND filters that are multicoated or you will suffer from more internal reflections, and increased refraction. Always use a lens hood, just as you would without any filter. When looking at brands, consider Heliopan, B+W and Breakthrough. There are many, but these are the only ones that I would recommend.
The Variable Story
There are two kinds of variable NDs. The first are really expensive and excellent. Everything else is junk and should be avoided at all costs. If the variable that you are considering is not at least twice the price of a really good polarizer, it’s going to have a negative impact on your image in at least increased moire and a tendency to a nasty green colour cast. Variables are not really cost effective, impede the use of a lens hood (because you have to rotate the outer ring) and you can likely get a couple of really high end fixed density filters for the price of a decent variable. I own and use the Tiffen variables, but only for video and for stills will still occasionally use my Heliopan variable. I have not found any others worth consideration.
Slide In Filters
Back in the 80’s, the Cokin brand became widely known. These, and clones like Filtek, were predominantly effects filters and have never been known for optical excellence. Cokin remains available and are decent as well as inexpensive. They are not multi-coated so really are a rare use option. As with all filters, multicoating is recommended. There are now multicoated filters from Lee, the name probably best known, but excellent options come from Haida Filters and in my opinion, the Formatt Hitech Firecrest NDs are the best out there. Some users have found the Lee Big Stopper to produce significant colour shifts. I have not seen this myself, and have found no shift with either Haida or Firecrest.
You should also consider the durability of the mount system. Some are made entirely of plastic, some are metal. If you are using ultra wide lenses that do not take front mount filters, you will need a special mount and be aware that some of the “supported” mounts still produce vignetting at the widest focal lengths. The filter holder that you choose should provide the option to hold more than one filter, but be reducible in the field to a single filter to avoid vignetting. It is my recommendation to buy the largest filter that you will ever need and use the proper mounting rings so you are not buying multiples of the same filter to fit different lenses.
Special Purpose NDs
Spot NDs are very rare and typically only used on large format film lenses. The graduated ND is quite popular to help tame dynamic range in landscape shots. The Formatt Hitech grads come in hard edged, soft edged and reverse options and are rectangular to you can adjust where in the frame the transition occurs. The demand for multicasting is critical here because you will have a clear section where the chance of internal reflections being visible is highest.
ND Reference Table
I’ve provided this table for your reference to help in your purchase plans
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I'm Ross Chevalier, thanks for reading, watching and listening and until next time, peace.