NISI ND filters review & comparison to HAIDA
Landscape and Wildlife are my favorite photography subjects and those in which I specialize. In landscape photography the use of filters is quite common, as a landscape photographer I've had much experience using variety of filters by several brands like NISI, Haida, LEE, Hoya, B + W and more. The main filters used for landscape photography are ND filter which reduce the amount of light going through the lens to the camera. ND filters allow to shoot at slower shutter speed to get effects of motion smearing (clouds, water streams, waterfalls...). The second common filter is the Graduated ND filter which helps balance exposure differences between the sky and the foreground. Another common filter is the polarizer used primarily to control light reflections.
During November 2017 I received the NISI ND64 filter and the NISI V5 pro kit with the built-in polarizer and went out to test them. The photos you will see in this review were taken in field conditions in relatively stable (but not permanent) lighting conditions. Below are the impressions from my personal experience with them.
subjects i tested through the review
nisi V5 pro kit
The kit contains 4 adapter rings (67 mm, 72 mm, 77 mm, 82 mm), the filter holder and the polarizer. The range of adapter rings is good and suitable for a variety of lenses, especially to most common wide-angle lenses used in landscape photography. The rings are made of high quality aluminum. The filter holder stands out in its quality from other holders in several things:

Reference photo without Holder or filters | 16mm, ISO100, F/16, 1/125 seconds

Photo with the V5 pro Holder mounted, no apparent vignette or darker corners | 16mm, ISO100, F/16, 1/125 seconds

Photo with the V5 pro Holder and polarizer mounted, no apparent vignette or darker corners | 16mm, ISO100, F/16, 1/40 seconds
NISI 100X100 Nano IR ND64
This is a glass filter and it comes in a leather pouch protected by rice paper. It has sponges on the back that prevent light leak and reflections into the lens. The main issues I tested while reviewing the filter were those ND filters have the most drawbacks: corner darkening and color cast. I compared the NISI IR ND64 filter with its main competitor in this quality level - The Haida ND64 (I did not have the chance compare it to the Haida "Red Diamond" series). All sample images were taken in RAW and did not undergo any post processing other than White Balance synchronization.
Color neutrality and color cast
The NISI filter is by far the most neutral filter I have ever used. I have previously worked with LEE, Hoya and Haida filters. I have no doubt this filter shows the least change in color. In the photos I took during the test I noticed the filter has a very small color shift to purple, but it is barely noticeable and can be easily corrected in post processing. The Haida filter has a fairly clear color shift to blue, the shift is more significant and visible from the NISI filter, but is by all means not terrible and can definitely be corrected in posr processing. I did not compare the corresponding LEE filter to NISI or HAIDA, but from previous experience with the LEE 10 stop filter and other reviews on the web about the LEE ND64, it is very clear it has a strong color shift to blue and that it is not as good as NISI or Haida.

Reference photo without Holder or filters | 16mm, ISO100, F/16, 1/125 seconds

Photo with NISI ND64 mounted | 16mm, ISO100, F/16, 1/3 seconds

Photo with HAIDA ND64 mounted | 16mm, ISO100, F/16, 2/3 seconds
vignette and corners darkening
I didn't notice any corner darkening using the NISI ND64 filter. It is a real advantage and it shows there is very good uniformity in all parts of the filter. With the corresponding Haida filter on the other hand, I did see visible corner darkening. It is important to note that it is not terrible, but it does require post processing that can sometimes reduce the image quality a bit.
filter density (darkening) accuracy
The NISI filter seems to be very accurate, it reduced the exposure a bit less than 6 stops, around 5.6 stops. The Haida filter was darker, about 7.3 stops. There also seems to be more darkening in the corners and less in the center in terms of darkening uniformity of the Haida filter.
Sharpness
I didn't notice any changes in the image sharpenss using both NISI and the HAIDA filters. This is very important and shows that the quality of their glass and optics are excellent.

Reference photo without Holder or filters | 16mm, ISO100, F/16, 1/125 seconds

Photo with NISI ND64 mounted | 16mm, ISO100, F/16, 1/2 second

Photo with HAIDA ND64 mounted | 16mm, ISO100, F/16, 1/2 second
conclusions:
It seems NISI certainly did excellent research and good work about the ND filters market. Their holder is comfortable, high quality and innovative. It seems that apart from the Haida holder it has no competition at the moment for those who are looking for a system that can be used with a polarizer along with ND filters. The NISI ND64 filter was very impressive and surpassed the rival Haida filter in almost all parameters (sharpness, corner darkening, color cast, darkening accuracy). It's also important to talk about costs - NISI's products are not cheap, but they are definitely top quality and worth it. For those who are looking for the best ND filters and can afford it, NISI are the best in my opinion. For those looking for very good filters (a bit less than NISI) at a more affordable price, I would recommend using Haida.
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