Neutral Density Filters
During our National Park Photography Master Class and First Horizons workshops, we are often using filters for photography- specifically neutral density (ND) filters. To be clear, these are full neutral density filters, not gradients which are sometimes used to darken landscape skies.
Fixed vs. Variable
There are two types of ND filters, fixed and variable, and you can buy them as screw-on filters for your lens or with a lens holder that mounts in front of your lens. (Here are some examples: Variable Filter, Fixed Filter) I use both, but I find that the fixed is more precise than the variable. With some practice, a well-made variable can render excellent results as well.
The issue with variables is that because you as the operator are turning the vari ring on the filter it can be difficult to achieve a precise setting. At some settings, you can develop an exposure artifact that looks like an imprecise vignette or create a dark X in the middle of the image.
The fixed neutral density filter evenly blocks some of the light so you can reduce the light during the day and create a longer exposure duration, which is very handy for waterfall images and skies in some cases. Fixed ND filters have a specific density, so you can quickly adjust your exposure settings based on adding the ND filter to the mix.
The variable neutral density filter is a different horse altogether because it is made from two polarizing filters, one stacked on the other. The rotation of the two filters relative to each other provides the variable amount of light-blocking capability, and while there are marks on the outer ring, these are not calibrated adjustments. As we know from using a single polarizing filter, we can introduce an X effect into an image at some settings, and this will occur with a variable as well.
Which One Should You Use?
I use and teach our workshop participants to use both. The variable is flexible but requires more careful application whereas the fixed units have no challenges. Variables have a range of 2 to 10 stops and fixed come in ranges of 2, 4, 6, and 8. With a filter holder, you can stack your fixed ND’s to achieve other combinations of neutral density.
ND filters are a useful creative tool, and every landscape photographer should have them in their camera backpack.