K&F Concept 77mm Slim Variable ND Neutral Density Adjustable Fader ND2 to ND400 Lens Filter
Price: $24.99
A variable neutral density filter allows you to adjust the amount of light reaching your sensor without having to carry around a bag full of screw-on filters, or expensive and time consuming filter holding systems. If you have been in the market for this type of filter you know that this one is on the budget end of the spectrum, and it has its drawbacks. When purchasing this filter you are agreeing to make a compromise, but as you will see in reviews of much more expensive models, that compromise is ubiquitous across the market on variable neutral density filters. I would also recommend that you consider standard screw on ND filters over variable options for still photography.
My overall assessment is that this filter will be that it is completely adequate to all but the most demanding landscape photographers, offering good functionality at an affordable price. However, I am going to start with the negatives so that everyone is aware.
The problems associated with variable neutral density filters fundamentally arise due to how this filter functions. The filter is comprised of two polarizing filters, and by rotating the front filter you can increase or decrease the amount of light reaching your sensor. Now, at the most extreme setting this filter will exhibit the undesirable effects of this design in quite a pronounced fashion. The Black X:
Now do not be too alarmed this effect is only present at the most extreme setting. When used at less extreme settings this filter exhibited a very slight loss of sharpness that is only apparent when zoomed in to 1:1 magnification and about a 150K shift in white balance towards yellow that is easy to correct for in post as long as you shoot in the raw file format. The only other negative I noticed is that it is that when you adjust the filter, the dimming effect is quite slow at the beginning of the range (stays at 1 2/3rds stops from minimum to about a third of the way through the rotation), but then dims at a much greater rate at the higher end of the range. It would be nice if the dots on the filter correlated to the effect it would have to reproduce desired results.
Methodology:
Now my testing methods would not hold up to scientific inquiry, but I made certain to follow certain procedures to ensure consistency between examples.
Images captured on a Canon EOS 6D
Lens used was a Sigma 50mm f/1.4 Art
Tripod used with center column down
Cable release
White balance set to 3200K
Raw file format
IS0 100, f/3.2
Base image shot at ISO 100, f/3.2, 0.5 seconds
White balance was corrected in Adobe Lightroom in the shots below
Some exposures have been adjusted for consistency (0.3 stops or less, unless otherwise noted as in the last image)
Base Image (0.5 Seconds):
2 Stops (2 Seconds):
3 Stops (4 Seconds):
Four Stops (8 Seconds):
Five Stops (15 Seconds):
6 Stops (30 Seconds):
7 Stops (in time value 60 Seconds): Adjust to 8.5 stops when considering post processing adjustments (150 seconds):
(Camera meter was thrown off at this level of darkening, exposure was pulled up in post by 1.5 stops)
So is this a highly recommended purchase? Of course not, it’s a compromise. Is this a purchase that has quite good performance for the price? Yes it is as long as you have realistic expectations and an understanding that you will get a black X in your frame if pushing the filter to stronger settings. So, I have covered that variable ND filters are an option if you do not want to carry around a rectangular drop-in filter system or a bunch of screw on filters, and I would argue that overall K&F’s iteration is worth far more than its selling point and a decent option for beginners to professionals on a budget that want to conveniently experiment with long exposure photography, or wide aperture photography/video in bright conditions. If you are serious about using filters, you will quickly replace this one.