variable neutral density filter

Testing the FOTGA 58mm variable neutral density filter

Does this thing work?

I’m not totally sure when/where I got this filter. I see that I bought a “neewer 58mm ND Fader filter” from Amazon back in November 2012 for $11. Maybe this is what they sent me back then and I just wasn’t paying very close attention to the brand. Anyway, I never used it all that much and when I did, I had some concerns about focus and sharpness. But I never really did any testing.

This week I started putting together a presentation on using speedlights and other flash units outdoors and needed to use an ND filter for some of the examples. So, I brought out this Fotga 58mm variable ND filter and decided to do some tests with it. I wanted to see a couple of things—focus and color shift. At $11, I wasn’t expecting the color to remain neutral. Many so-called neutral density filters, even at much higher cost, aren’t all that neutral. I’ve seen many reports of green, blue, and magenta color casts in ND filters. I had no idea what to expect in terms of sharpness. A variable ND filter is basically two polarizer filters combined in a rotating mount, so there are 4 more glass (or plastic) to air surfaces between your camera and subject. So I expect to see some effect on the sharpness of the image.

So, I set up a chart made up of radial lines and a Passport Color Checker with a Canon EOS M5 mirrorless camera and 85mm f/1.8 lens. The camera was mounted on a sturdy tripod and I used a cable release. The camera was set to ISO 100, aperture priority mode, tungsten white balance (I used the 250 watt quartz modeling lamp in an Einstein flash with a Photoflex Medium Silverdome NXT softbox). and f/5.6 (for something in the middle of the aperture range). I actually tested at a variety of f/stops, but am presenting the 5.6 images here, the others were similar). The filter was set to the 4th tick mark on the ring and the exposure time without the filter was 1/13 second and with the filter was 1/4 second (1-1/3 stops different).

Let me start by saying that the build quality on the filter seems pretty good. The filter elements spin smoothly, but are not so loose as to accidentally change the settings. The filter appears to be plastic. It easily screwed into the 58mm threads on the 85mm lens.

Yes, there is a color shift when using this filter. It was consistent at all ND settings of the filter, from Min to Max. There was a -200K to -300K shift in temperature and a +9 shift in tint. That is, the filter warmed up the image by 200-300 degrees Kelvin and added some green. Both easily corrected in the raw files in Lightroom.

Sharpness suffered slightly. I don’t know how to measure it, but you can see crops of the same section of the chart below. Without the filter you can clearly see the texture of the paper the chart is printed on. With the ND filter it is harder to see the texture. Still, for $11…

One interesting thing is the markings on the filter. I consistently noticed that between the MIN setting and the 4th hash line on the filter there was no change in exposure. That is at Min and f/1.8 and 4th line at f/1.8 the camera metered the same 1/40 shutter speed. At Min and 4th line at f/4 it remained at 1/8, and at f/8 it remained at 0.5 seconds shutter speed. Moving to the 8th hash line and to the MAX setting gave expected changes in the exposure.

Someone asked to see the comparison at f/1.8, so here it is:

Conclusion? This works for me in the limited uses I have for an ND filter. It should work great for cotton candy waterfalls, even providing a little warmth (like adding an 81a filter for those of you who still remember filters in the film age). In the studio, I have a strobe that can go way down in power when I want to photograph wide open, so I don’t need it there. I don’t do much outdoor fill flash with a studio strobe right now. And if I do, I will get a strobe that offers high speed sync, such as the Interfit S1, to handle wide open apertures instead of dealing with the hassles of ND filters in the field (dark viewfinder, auto-focus not working, trial-and-error metering and exposure settings, etc.). If I was doing critical work or client work that required a neutral density filter I would be looking at the Singh Ray filters, but their cost is a little bit more than $11 (more like $340 - $390 for a 58mm filter, depending on choice of standard or thin ring size). 

Other reviews of the FOTGA filters mention eBay and long delivery times. It looks like they are also available on Amazon, and I just ordered a 72mm for $13.99 to test on some telephoto lenses. Amazon says it should arrive in 2 days. I will add more to this post when I get to test the 72mm version.

Anyone else using these inexpensive variable ND filters?

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John Cornicello