I have been looking at variable Neutral Density filters for a while. I use ND filters when shooting flowing water and occasionally for other shots where I want to reduce the DOF and the light is too bright to open up all the way. I have a set of 3" square resin ND filters that I bought about a decade ago but they are cumbersome to carry around and use. I also find that the higher density ones impart a significant brownish color cast to my images. No biggy when working in B&W but a nuisance with color images.
Enter the K&F Concept Variable ND filter that combines a whole range of ND filters into one. Most of the ones I looked at were rather expensive (as in three figures) which is a bit much for a retired guy who isn't making money from his photography and has only occasional use for an ND anyway so the $19.99 price of the K&F filter on Amazon looked like a real deal especially since I could return it if it was a bust. It isn't. I'm sure that in lab testing a $100+ filter might prove to be somewhat better but for my limited needs, the K&F will do just fine.
My test was pretty simple. I mounted my EOS M3 with the 18-55mm on a tripod and shot a photo of my barn and tractor (nothing artistic or even pretty, it is just a test). The ISO was set at 100 and the mode was aperture priority. I then mounted the filter and set it on the "MIN" mark and shot another. I took a series of photos moving the filter setting 2 dots toward the "MAX" mark between each except for the next to the last when I moved it all the way to MAX. As you can see I had a problem there. It shut down the light unevenly and so severely that the exposure went all the way to 25 sec. Finally, recognizing that was a problem, I set it back to the dot just before MAX and shot one more. The results are below.
If you click on the image above, you can see it full sized as it appeared on my screen.
Thoughts on the results: I was concerned that like my square resin filters I might get a color cast. I didn't. That is the first plus. The filter appears to be very well made, is small and convenient compared to the square resin filter set and the associated holders. Another big plus. Those two things mean I will actually carry and use this more than I ever did the set of square filters. The difference in exposure times from 1/25th second unfiltered to two and a half seconds is more than adequate for my needs. I just have to remember to use the dots and not set it on MAX itself. Examining the images at 100% I saw no degradation of the autofocus between the unfiltered image and the others. When I first mounted the filter the setting marks were on the bottom but because the front turns 360° freely (it is truly variable, not click stops or detents) I was able to turn it so they were on the top and because of the way VNDs work it was fine. I will add my own little white indicator mark to the opposite side of the back ring.
Variable ND filters work by crossing the planes of two polarizers. The nearer the planes get to 90° to one another, the less light gets through. Theoretically, if the light was perfectly polarized and the planes were at precisely 90°, no light would get through. I expect that the X on the MAX setting is the result of small imperfections in the filtration and/or the planes of polarization being slightly off. I'm not going to spend hundreds on some of the more expensive VND filters to find out whether they do the same.
If you are in the market for a variable ND filter and don't have big bucks to throw at it, the K&F Concepts filter is a winner for the photographer on a budget IMO.
No one has offered me anything for doing this review. The Amazon seller did ask that I review my purchase on that site. This is my honest assessment of the product.
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