A variable neutral density (ND) lens filter is easily screwed onto a camera lens and consists of two opposing polarising filters. The front element can be rotated to control the level of light that enters a camera’s lens, without affecting colour. A variable ND lens filter is very flexible, as it lets you alter light exposure between around 2 (ND4) and 9 stops (ND500), without needing to change the filter from your lens.
There are plenty of situations where you might wish to use a variable ND lens filter, and as is the case for ND filters in general, it holds great appeal for any genre of photography. In particular, outdoor photographers who deal with the effects of bright light conditions find this filter a godsend.
If your filters get dirty quickly, particularly if you’re into coastal photography, a variable ND lens filter is very useful. With this filter, you only have to clean one. However, if you swap and change other density filters, there would be much more cleaning involved! This saves time, letting you fully concentrate on getting your exposure settings spot-on, just at the right crucial moment.
By simply rotating the filter, you can slow down shutter speeds to create blurring effects. This is great for shots of moving water, vehicles or people. Or, you can make something stand out from its background by widening apertures to produce a shallow depth of field.
With a variable ND filter, you can adjust your exposure while you shoot, as the lighting changes in a scene. This is commonly referred to as ‘riding iris’. You rotate the filter to change the exposure as light alters, keeping the same f-stop and depth of field.
One scenario where variable ND filters really come into their own is during film or video shooting. Since you have less flexibility to alter your settings in cinematography, this filter is really handy for allowing quick and precise exposure adjusting. In fact, if you need to film on a really bright day, you would be lost without this filter.
If you use a variable ND lens filter with slow shutter speeds, it’s worth sticking your camera on a tripod. This helps to keep it stable and prevents any blurry images.
You can easily stack ND filters to account for different levels of light. However, many photographers find that using a variable ND lens filter gives you more precise control over your end results. It also allows for easier experimentation. Plus, it’s pretty straightforward to use, and having just one filter to carry about offers convenience for the travelling photographer.
One thing to note when using variable ND lens filters is that if you rotate them beyond their minimum or maximum densities, you’ll get what looks like an X mark on your image. This will happen no matter what the quality of your filter, as the angle of light becomes so varied between the two layers of glass. To avoid this occurring, keep within the density parameters.
Another undesirable effect of using a variable ND lens filter, which is actually dependant on its quality, is that colour casts in your image can sometimes occur. For this reason alone, always buy a decent quality variable ND lens filter, and look for a thin one if you want to slash the risk of vignetting on wide-angle lenses.