Today’s Question: I enjoyed your [webinar] talk about lenses. You talked about solid (rather than variable) neutral density (ND) filters. Is there an issue with using a variable ND filter?
Tim’s Quick Answer: There are two key challenges with a variable neutral density (ND) filter. First, it can be difficult to achieve a precise exposure adjustment with a variable ND filter. Second, variable ND filters can cause an “X” shaped exposure artifact similar to vignetting.
More Detail: A solid neutral density filter basically serves as sunglasses for your lens, evenly blocking some of the light so you can (among other things) achieve a longer exposure duration. A given solid ND filter will have a specific density, so you can easily adjust your exposure settings based on adding the ND filter to the mix.
A variable neutral density filter typically achieves its variability by stacking two polarizing filters together. The rotation of the two filters relative to each other provides the ability to “dial in” a variable amount of light-blocking capability.
A variable ND filter typically has marks that can be used as a guide for rotation, but those marks won’t provide you with a calibrated adjustment of the resulting density for the filter. This can be a challenge when it comes to adjusting your exposure settings.
Perhaps more importantly, when using a variable ND filter you may see an artifact similar in concept to a vignette, but in the form of an “X”. The central area of the image will be properly exposed, but with a somewhat obvious darkening at the top, bottom, left, and right, forming something of an “X” shape outline. This is similar in concept to the exposure gradation you might see with a polarizing filter, but with an appearance that can be a bit more problematic.
So, my personal preference is to only use solid ND filters rather than variable ND filters. To be sure, a variable ND filter provides great flexibility and utility, but to met he challenges are more significant than the potential benefit of leaving a few solid ND filters behind. So I personally travel with a few solid ND filters, and don’t use a variable ND filter.