Today’s Question: I’m in the market for neutral density (ND) filters. I noticed a Variable ND filter that you can rotate (much like a polarizing filter) and dial in anywhere from ND8 to ND2000 [three stops to eleven stops of density]! Gimmick or purchase worthy?
Tim’s Quick Answer: While variable neutral density filters certainly aren’t a gimmick, I don’t really recommend them because they be a little tricky to use and can produce a form of problematic vignetting within the image.
More Detail: I absolutely understand and appreciate the appeal of variable neutral density (ND) filters. Instead of carrying several solid ND filters you can use a single filter that could run the range between about two or three stops and as much as ten or eleven stops. However, there are some problems with variable ND filters that cause me to prefer solid ND filters.
The most significant problem with variable ND filters in my view is that they can produce problematic vignetting and crosshatching artifacts in images. In some cases, for example, you might see a darkened “X” shape across the photo. These visual artifacts can be very difficult to correct later in your workflow.
In addition, variable ND filters can be a little challenging to work with. While there are marks on the outside of the filters that provide some sense of how much you’re blocking the light, you don’t know exactly how much light is being blocked. At lower density settings you can often just depend upon the metering in your camera. But at greater density settings the metering may not be reliable.
I very much prefer being able to configure a good exposure, and then add a solid ND filter of a known density and adjust the exposure settings accordingly. This can’t really be done with a variable ND filter.
So, I very much prefer carrying a couple solid ND filters rather than a single variable ND filter. Note, by the way, that while a variable ND filter generally offers a pretty good range of stops of light it can block, I only find that I need a couple of solid ND filters.
I typically carry a 6-stop (https://timgrey.me/6stop) and a 10-stop (https://timgrey.me/10stop) solid ND filter. If I need the equivalent of a filter with less density I can simply raise the ISO setting to compensate.