A neutral density (ND) filter reduces the amount of light entering your lens. Since photographers typically want more light, blocking some light sounds like a crazy idea. But for long exposure photography when you want to purposefully introduce motion blur, or for when you need a longer shutter speed for video or flash photography, neutral density filters are often essential. In this article, I’ll be reviewing one such filter: the K&F Concept Nano-X variable 1-9 stop ND filter. This is a relatively affordable filter, costing $73 in the 58mm size that I tested.
Table of Contents
Build Quality and Features
The K&F Concept 1-9 stop Nano-X is made up of two pieces of glass rotating against each other. As a variable ND filter, it allows you to control the level of light reduction, which is proportional to the difference in the angles of polarization of the two glass elements.
This particular filter comes in a nice pouch, and it has a small ribbon on the side that aids in extracting the filter. It’s a much more practical solution than some of the plastic cases that come with other filters that often pop open. In the field, it can be difficult to juggle filters, so I appreciated this convenience.
Adjusting the filter’s density is easy, and can be done with the orange tab that sticks out the side. It’s a great way to turn the filter if you have gloves on, but it does prevent you from using a traditional lens hood. However, the orange tab can be taken off with a small Torx screwdriver if you’d prefer to use a hood. The filter itself has a small arrow that points to gradations on a scale, making it easy to adjust the level of darkening.
Vignetting Performance
Many filters add a bit of additional vignette, which is usually pretty easy to correct in post. I tested the filter on the Olympus 12-45mm f/4 PRO lens at 14mm and 45mm to get a rough idea of the vignetting, and I also compared the filter against the bare lens. By dividing the luminosity of a photo with and without the filter, I could get an approximate idea of the vignetting of this filter. To see if the density of this variable ND filter mattered, I tested it at both the 2-stop and 5-stop density levels:
This test is pretty informal, but shows that the vignetting is minimal at 45mm (90mm FF equivalent), and a bit more noticeable at 14mm (28mm FF equivalent). Also, rotating the ND filter for different densities did not seem to change the vignetting in a meaningful way.
To further test the vignetting, I compared the K&F filter against a fixed neutral density Hoya filter that I already owned. They seem to have roughly equal vignetting, accounting for the variance of the measurements:
So, the vignetting at wider focal lengths seems about typical for a neutral density filter. It’s not too strong and goes away in post if you just add a bit of extra correction. I also shot a lot of video with the filter at a variety of focal lengths, and I didn’t run into any trouble with vignetting with the footage.
One thing you do have to keep in mind is that this filter uses two combined polarizing filters in order to achieve the variable effect. Because of this, at some wider focal lengths, you can get slightly uneven vignetting across your image, which looks sort of like this:
I was curious if this asymmetry would show up in my measurements, and it did. The interesting thing is that I didn’t measure any significant asymmetry at 45mm (95mm equivalent) but measured some at 14mm (28mm equivalent). However, the asymmetry was only visible when using the filter at a stronger density level. It showed up with 5 stops of ND filtration on this filter, but not with 2 stops. You can see those results below – note, the red lines blocks the virtually identical blue line:
This is a limitation of all variable neutral density filters, so it can’t be helped. It is mostly not noticeable and correctable, but once you are around 9 stops, then it will be noticeable in real-world images. Thus, I’d recommend keeping the filter at 8.5 stops at the most. This makes sure you don’t get too close to the “X” effect, where there is significant unevenness across the frame.
Here are two sample photos taken with fairly even, controlled lighting at the five-stop setting after lens correction so you can see there is really very little added vignetting at the five-stop setting:
Also, with the polarizing glass, you may also see a complete darkening of already-polarized light, such as the light coming from computer monitors. Again, this is not a fault of this filter but just a limitation of the polarizing glass. That being said, if you’re filming or shooting in a place with computer screens, chances are you’ll be indoors and won’t need an ND filter anyway.
Color Shift
Another factor in choosing a filter is color shift. Some filters add a bit of color, while others are neutral. Ideally, the filter should be as close to neutral as possible. Again, I made an informal test by taking a picture of a PDF color chart at a fixed white balance on my screen, with all lights off. Here’s the result:
The Hoya Filter is fairly neutral, but there is a slight warming in the K&F concept filter. Setting the white balance taking from the same spot in all three images, I measured the percentage correction required for both white balance and tint:
This shows that the K&F concept warmed the image a bit (around 500K) and shifted the image a bit towards the magenta end of the spectrum. Overall, the effect was slightly stronger than the Hoya filter. That being said, after correcting all three images for white balance, it was very difficult to tell the difference between the images, and in practice the shift is fairly slight. Here are the waveforms of all three images after correction, showing very little signs of permanent damage from color shift:
Overall Impressions
Practically speaking, I feel that the K&F Concept Nano-X Variable ND filter is a great filter. In shooting many hours of video outside with it, I ran into few actual problems. True, it does have a slight amount of color shift, but that’s easily corrected. Moreover, it’s not easy to find an ND filter on the market that has such a wide range of 1-9 stops. The ones that do exist either have a narrower range of stops or are more expensive, sometimes significantly so. (The K&F filter is $73 in the 58mm size that I tested.)
So, who’s this filter for? If you’re a photographer who needs a wide range of ND stops, or a person who shoots a lot of video, I think this filter would be great. To my knowledge, it’s the most practical and affordable filter that has good performance. It’s true that fixed filters have a few advantages, but they are often impractical for those who shoot video. Even if you only shoot stills, they take more time to use.
If you’re interested in buying this filter, check out either the B&H or Amazon links here. Buying gear using our links helps support reviews like this at no extra cost to you!
(Don’t forget to select the correct filter size.)
