(Probably) The Biggest Variable Neutral Density Filter You've Ever Seen

Here at Fotodiox, we offer all sorts of direct solutions to problems you may approach on a project. The right tools exist for the right occasion, but I don’t mind a good DIY solution either. A couple of months back, I stumbled onto this video:

Having a variable neutral density option lightens your bag by removing the need to carry multiple grades of neutral density filters around with you. It’s also suitable for run-and-gun video shoots, making it an indispensable tool for filmmakers:

1/50 f/1.8 ISO100, without Neutral Density

1/50 f/1.8 ISO100, with Neutral Density

As you can see, having the neutral density filter brings details back into the highlights. Rather than switching out filters until you find the right one, a variable neutral density filter allows you dial in the perfect level to suits your needs on the fly.

Alternatively, I thought of our ND Throttle line of adapters. Out of the box, these adapters offer two to eight stops of neutral density with the turn of the ring.

Then it clicked: it’s not like we offer an ND Throttle for every combination out there. In some cases, an ND Throttle is impossible for the lens-to-camera combination you’re using (spoiler alert: if your camera and lens have the same mount, no way; it’s a focal flange distance issue). A variable neutral density filter on the front of the lens is a pretty cool solution.

If you don’t feel like watching the video above, we’ve broken down the instructions below:

1) For starters you’ll need two circular polarizing filters of the same size:

For this example I needed 49mm filters to use with our Canon 50mm f/1.8 STM lens.

2) Take one of the filters, remove the retention ring that secures the glass:

A lens spanner wrench makes this a lot easier. I used two small screwdrivers on my first try and it was not the best choice. Don’t be like me. Don’t scratch your filters.

3) Flip the glass over, and reinstall the retention ring:

Gloves are probably a good idea. Last thing you'd want is fingerprints on your CPL.

4) Screw the modified CPL onto the untouched CPL.

5) Adjust to your heart’s content.

Between the lightest and darkest points of the filter adjustment, I was able to get about nine stops of darkening, equating it to an ND512 at its darkest.

It would be pretty convenient if all the lenses we use here need 49mm filters, but that’s not usually the case. As stated in this article from our friends at B&H, it’s best to use a filter that matches the largest thread diameters of the lenses you have; anything else can use that same filter when combined with the appropriate step-up ring.

In my personal lens arsenal, I have lenses with filter threads ranging from 52mm to 77mm. The logical thing for me to do would be to repeat process above with a 77mm filter then use step-up rings as needed. However, we offer a bigger size than 77mm and I felt like getting a little crazy, so I decided to use our WonderPana XL 186mm round filters, which comically large compared to the 49mm filter.

The extraordinarily large size of this filter means that we should be able to open up the option for variable neutral density filters for our WonderPana system (kind of cool, since we offer plenty of filters for ultra-wide angle lenses, but none of them are variable neutral density filters).

As I went outside to try my new toy, I noticed something most companies that manufacture variable ND filters will warn you about. Once you darken a variable ND filter enough, you will reach a point where a cross pattern occurs:

Since it starts off rather subtly, I’ve highlighted the cross pattern in the image above. When you know where to look it is easy to see the discoloration on the opposite edges of the frame. This is also about as mild as it gets.

When it gets bad, it gets really bad.

Once I darkened the scene by one stop (and compensated the shutter speed by one stop), I was already getting the uneven tone on the top-left and bottom-right corners of the frame. It seems this DIY solution does not work for this lens, and likely not for other ultra wide-angle lenses, but it turned out to be a fun experiment and learning experience.

With this solution being a dead-end, I returned my attention to the ND Throttle line of adapters. While we can’t use our Canon 11-24mm on a Canon camera with the ND Throttle, we can use it on a MILC (Mirrorless Interchangeable Lens Camera), like the Sony E-mount sytem. This makes the most sense for me because I can take advantage of the full-frame sensor in our A7RII and we offer a Fusion variant for it. With the Fusion ND Throttle I still get full electronic control of my aperture, as well as auto focus. After some testing, it seemed to work fine:

1/60 f/4 ISO 1600, using ND Throttle

Takeaways from this experiment? Variable neutral density is possible for ultra wide-angle lenses, but not as a front-mounted filter like we can use on most prime and telephoto lenses. Variable neutral density filters work well behind the lens, but this positioning does rely on adapting compatible lenses to a camera body with a different lens mount.