Neutral density filters can be a very expensive investment for photographers and choosing the correct system to begin with can save a lot of money over time. Neutral density filters are available in varying densities, graduations, sizes and shapes. The two major shapes of ND filters are circular and square. Circular ND filters are designed to directly screw on to the front of lenses that have a screw thread. Square filters require being inserted into a holder that will typically clip onto an adaptor attached to the front of a camera lens.
While personal preference may play a big part in whether you decide to purchase circular or square ND filters, understanding the advantages and disadvantages of each will help you make the best choice for your long term photography needs. Taking into account whether you would require the use of multiple filters at any one time, the types of images you wish to produce and the environments you shoot in, your long term investment into different lenses and your overall budget will all factor into whether a square or circular neutral density system is best for you.
Square Neutral Density Filters
Square ND filters are generally a reference to those that require a filter holder of some type, rather than screwing directly onto the front of a lens. Square ND filters can be both square and rectangular in shape, and come in variations of sizes such as 75mm x 80mm, 75mm x 100mm, 100mm x 100mm, 100mm x 150mm, 150mm x 150mm, 150mm x 170mm, 180 x 180mm and 180mm x 210mm. The choice of whether to use 75mm, 100mm, 150mm or 180mm filters will depend on what lenses you have or plan to have in your lens collection.
Lens threads of all sizes are catered for with square ND filter systems. Even lenses without a filter thread that have built-in lens hoods will most likely have adaptors to allow a filter holder to be attached to them. Rather than screwing on, some lens adaptors such as those to suit a Nikon 14-24mm f/2.8 or Tamron 15-30mm f/2.8 will slide over the front of the lens hood and “tighten” via a collar to keep them in place securely.
Square ND filters can be rotated using the filter holder, which allows you to align them with the horizon line and other elements of a scene. Graduated ND filters can be inserted and moved up and down as well, which gives great flexibility in getting your overall exposure correct in the camera, rather than relying on post-processing.
Filter stacking using square ND filters will allow you to get more creative with your photography by using a combination of filters to both slow down your exposure time using filters such as the 6 stop to 10 stop (and many others) in combination with graduated filters. The square filter systems in the NiSi range are designed to also allow for circular polarisers to sit behind (closest to the lens and camera body) other ND filters while still being able to rotate them using the built-in cog wheels to produce the best polarisation result.
Unless a specific lens requires a larger size ND filter system due to either its wide focal length or because of the non-standard thread, a single size filter setup should suit the majority of lenses in your camera kit. This makes a square filter system a more versatile setup, and potentially a better long term investment if you require the use of multiple ND filters. 100mm neutral density filters are the most commonly used size and suit most major manufacturers lenses with thread sizes ranging from 49mm to 82mm, and focal lengths as wide as 16mm (sometimes wider depending on the specific adaptors). Lenses wider than 16mm, and those that have either no screw thread, bulbous or protruding front glass elements or a built-in lens hood will require either the 150mm or 180mm filter system.
Advantages of Square ND filters
Better long term investment to suit a bigger range of lenses – By using different thread adaptors to suit specific lenses, filter holders can be adapted to suit specific lens sizes, allowing for square ND filters to be used on a variety of lenses. Over time this can make a big difference in the cost of ND filters when comparing square to circular.
Filter Stacking – Stacking filters is far more effective and easier when using square over round ND filters. Most filter holders allow for either 2 or 3 ND’s in addition to a polariser. This allows the use of filters such as a 6 stop to 10 stop in addition to graduated neutral density filters.
Rotating filter holder – Filter holders can also be rotated and adjusted into varying positions making it much easier to line them up with horizons and other elements in a scene. Graduated ND filters can also be moved up and down in the filter holder for aligning them to the scene.
Lack of vignetting – Square ND filters are typically larger than the glass elements on the front of a lens and limit the impact of vignetting, even when stacking multiple filters together.
Disadvantages of Square ND filters
Bulkier Kit – When you consider all of the parts required for square ND filters the entire kit can be quite bulky, especially in the bigger 150mm and 180mm sizes. The filter holders, thread adaptors and filters themselves can take up quite a bit of space in your camera bag, and the weight can add up for those that are conscious of how much they are carrying.
More items to keep track of – The number of parts required when using square ND filters is far greater than using circular filters. A filter holder, at least one lens adaptor and the individual filters will all be required to be used.
Potential for light leakage – If filters are not inserted in the correct order, or if the rubber seal is damaged or missing from filters like the 6 stop or 10 stop ND, there is a chance of light leakage happening between the filter and the lens. This can have a major impact on image quality.
More expensive – Square ND filters are generally more expensive than circular ND filters. There is more glass required in the manufacturing process, and optical glass can be expensive to manufacture. The additional expenses of a filter holder and any adaptors to suit specific lenses can add up quite quickly.
Less durable – Because of their all-glass build, square ND filters are easier to break and less durable than equivalent quality circular filters. The surface area of square ND filters increases the chances of scratches, cracks and chips in the glass. Because there are no edges or threads to protect the filters, dropping them or not packing them correctly in your camera bag can lead to damage.
Circular Neutral Density Filters
Circular ND filters are round in shape and will have a thread on them allowing them to screw straight onto the front of compatible lenses. These are typically designed to fit a specific lens thread size ranging from 40.5mm all the way up to 95mm. When using a larger circular filter, step down adaptors can be used to fit them to smaller lens threads, but you cannot adapt a smaller filter to suit a larger lens thread. This means when purchasing circular filters it is best to consider the largest thread size you will require and use adaptors to match them to any smaller lenses you may have unless you intend to purchase individual filters for each thread size you require.
Round filters are available in variations like the 3 stop, 6 stop, 10 stop, 15 stop, circular polariser, UV filters, variable ND, ND + CPL multi function filters, graduated ND and the NiSi Natural Night Filter, which basically covers the full range of ND filters that most photographers would require.
Convenience and ease of use is one of the major reasons many people decide to purchase circular ND filters. Fitting them to the front of a lens is as simple as screwing them onto the existing threads, and once they are on there is no chance of them falling off (unless they have been fitted incorrectly by cross threading them). They are also very easy to store and transport and tend to be more durable than square filters due to metal threads that frame the edge of round filters.
While circular filters quite often have a screw thread on both sides allowing multiple filters to be stacked together, vignetting can often occur as a result of this. Round filters fit into the inside of a lens thread, and stacking multiple filters together increases the likelihood of the darkening of the edges and corners of your exposures, even at standard focal lengths. This makes them difficult to recommend if you require the use of multiple filters stacked together such as a 6 or 10 stop as well as a graduated ND filter. Round graduated ND filters can be rotated, but cannot be moved up or down due to them being fixed via the screw thread to the front of a lens. This makes them a less versatile filter if you do require the use of graduated ND’s.
Some lenses are incompatible with circular filters due to the lack of filter threads, bulbous and protruding front glass elements or fixed lens hoods. So over time if you intend to invest into different lenses you may find limitations to which lenses you can use with your existing circular filters.
Advantages of circular ND filters
Easy to set up – Circular ND filters are extremely easy to set up and use. Just screw them straight onto the front of your lens and you are ready to go. You may not even need to remove them from the lens if you can fit your lens cap onto them.
More durable – Because of the screw threads which are quite often made of titanium, aluminium, other metals or even plastic, circular filters are less prone to damage and breakage when dropped or packed in your camera bag. The edges surrounding the glass provide a protective barrier against impact. Less glass surface area and the lack of any corners makes them more durable.
No light leakage – With circular ND filters being screwed straight onto the front of a lens, there is no chance of light leakage being introduced between the filter and the lens.
Convenient to store and transport – Circular filters are very easy to store and transport, although when carrying a lot of them the weight and bulk can add up quite quickly.
Tend to be less expensive – When considering the cost of the entire ND filter system, circular filters tend to be cheaper than a square system. There is no need to purchase a separate holder and the individual filters themselves are quite often cheaper to purchase when compared to an equivalent square filter.
Variable ND filters – Variable ND filters allow for different strengths of density in a single filter by rotating them. The transition of density occurs smoothly which allows the photographer or videographer to adjust exposure length to the exact amount required for a given scene. This can give flexibility in choosing a single filter and varying its strength.
Disadvantages of circular ND filters
Some lenses do not have filter threads – There are quite a lot of camera lenses from different manufacturers that do not have a filter thread, which means circular filters cannot be screwed on to them. Quite often the bulbous front elements and built-in lens hoods also make it impossible to use circular filters.
One size does not fit all lenses – Circular ND filters are designed to screw onto a specific lens filter thread size. There are step up and step down rings that will adapt round filters to other sizes, but these do not always produce the best result. You cannot adapt a smaller filter to fit a larger thread size. You can use a larger filter on a smaller lens size using the correct adaptors depending on the type of ND filter you are trying to use.
Circular graduated ND filters be difficult to align – Due to the fact that ND filters cannot be moved up or down like square filters can be, aligning filters such as a circular graduated ND can require the camera to be moved rather than just moving the filter. This can sometimes mean making compromises in your compositions to suit the filter.
Vignetting is very common when stacking circular filters – Even with standard focal lengths, vignetting is very common with stacked circular filters due to the fact they fit inside the end of a lens, rather than over the outside of the thread. The more filters you use in a stack, the more noticeable this will become.