If you don’t have a variable neutral density filter in your camera case then you are seriously missing out! My advice, get one STAT! I typically use a UV filter to protect my lenses and a ProMaster CPL filter for some scenics, however, the new ProMaster HGX Variable ND Filter is one I just won’t leave my studio without.
Nuetral Density (ND) filters are basically sunglasses for your lenses; they diminish the light transmitting thru your lens. This lack of light transmitted permits you to use slower shutter speeds and wider apertures. Without using a ND filter you’d have to use faster shutter speeds with smaller f-stops to get a good exposure.
If you are a video shooter, then an ND filter will become your best friend. When you’re trying to use a relatively slow shutter speed for normal looking motion blur (i.e 2x your frame rate) and you are forced to use a sensitive ISO like 1600 (with log gamma), you can see how you may not be able to achieve a properly exposed shot, even at minimum aperture! We actually leave a ProMaster HGX Variable ND filter on our lenses at all times when shooting video and have ditched lens caps altogether! Set your shutter speed to 1/50th (for 24 fps), choose your favorite wide aperture for creamy depth of field, and then throw on a variable ND filter to dial in a perfect exposure.
You might be asking “why would I ever need this type of filter for photography?” It’s all about controlling your depth of field. Simply put, ND filters allow you to use longer exposures during the day without having to close down the lens. We’ve all seen those shots where the photographer has captured creamy, silky motion blur on water, skies, or traffic. While these shots are usually taken at night, an ND filter can help you achieve the shot during bright daylight.
How Do ND Filters Work?
A standard ND filter is of a single set density. So changing the density or how strong the filter is will require changing to a totally different filter. Filter strength can be reported a few different ways, but “stops” is usually the most intuitive, as most photographers are familiar with the term. ND filters are referred to as ND2, ND4, ND8, etc, which means it cuts light by a factor of 1/2 (1 stop), 1/4 (2 stops), and 1/8 (3 stops), respectively.
Variable filters can be adjusted, hence the name. The ProMaster Digital HGX Variable ND Filter ranges from 1.3 to 8.6 stops, which is great for video when your exposure is constantly changing depending on location. As you rotate the filter, it progressively attenuates more light. It uses two polarizing filters which when rotated, move out of phase with one another, cutting down light. Variable ND filters do have limitations though. When rotated to the extreme, cross type shading will occur. This is more noticeable with wider focal lengths. Also, you may notice a blue tint at extreme settings of the filter as well. These limitations are unfortunately inherent to all variable ND filters, just due to the physics of how they work. If you find you are pushing your variable ND too far and need more light stopping power, a work around is using a variable ND filter stacked on top of a standard ND filter for extra strength. Just be sure to use as high quality filters as you can, as stacking cheap glass can reduce quality.