Today’s Question: I’m not all sure what a “variable” ND [neutral density] filter is. I typically have used the ND filters that are either full ND filters (entire filter is one density) or split or graduated ND (one density on top that gradually or abruptly changes to another density on bottom). Is a variable neutral density filter a split or graduated filter? What is the mechanism for using a variable ND filter? Are they like circular polarizers?
Tim’s Quick Answer: A variable neutral density (ND) filter generally consists of two circular polarizing filters attached in a way that they can rotate relative to each other, creating a variable amount of light-blocking depending on the angle of alignment of the two filters.
More Detail: I often describe solid neutral density (ND) filters as being “sunglasses” for your lens. The primary purpose of this type of filter is to reduce the amount of light entering the lens so that the exposure time (shutter speed) can be extended. For example, I’ve often used a ten-stop neutral density filter to achieve exposures of up to about 30 second during daylight conditions.
Such a solid ND filter has a single density, meaning it blocks a specific amount of light. The density of the filter is often described based on how many stops of light get blocked, or how many stops you would need to extend the exposure time by once you have added the filter.
As the name implies, a variable ND filter enables you to dial in a variable amount of light blocking. These filters have two circular polarizing filters attached to each other in a way that enables the two filters to be rotated relative to each other. The angle of alignment between the two polarizing filters determines the degree to which light will be blocked. You can therefore use a single filter to achieve different effects, which would normally require a variety of solid ND filters.
One of the challenges of using a variable ND filter is that you can’t precisely determine how much light you are blocking with any given setting. Furthermore, especially with high effective densities, the exposure meter for your camera won’t necessarily be accurate. As a result, it can be a little tricky to get an accurate exposure with a variable neutral density filter.
That said, variable neutral density filters enable you to have a single filter that could effectively replace several solid ND filters.
You can see a sample of a variable neutral density filter here (just be sure you select the right filter size for the lens you intend to use):