Neutral Density Filter Guide: Everything You Need to Know About ND Lens Filters
Neutral density filters reduce the amount of light that comes into the camera to produce a filtered view that is comparable to that of looking through sunglasses. Neutral density filters don’t adjust the color of the light that comes in, which is why they are called “neutral” density lenses. While most cinematographers have neutral density filter lenses, few fully understand when, why, and how to use them appropriately in all settings. This neutral density filter guide provides everything you need to know about ND lens filters.
What Do ND Filters Do?
ND filters reduce the amount of light that comes into the camera, similar to the way sunglasses would reduce the amount of light that you see. Neutral density is the term used to describe the blocking of the light in a neutral way in which the color is unchanged.
Neutral density filters block the amount of light that is coming into the camera so that the shutter opens for longer to allow for the correct amount of light to enter. The result is moving things blur slightly, and stationary elements are crisper.
ND filters are used by cinematographers for a variety of purposes. Aperture and shutter speed are the two common adjustments that come to mind when discussing neutral density filters. Outside, the ND filter allows you to capture a shot that’s brightly lit without overexposure. The filter allows the cinematographer to produce a shallow depth of field and to use selective focus effects. Likewise, ND filters can also be used to slow the shutter speed of the camera to produce a blurred effect on anything that is moving.
ND Filter Numbering
ND filters are often numbered based on the strength or darkness level that is provided by the filter. Much like sunglasses provide different strengths of protection for our eyes from the sun’s harmful rays; ND filters provide different levels of light reduction. The numbers provided on the ND filter are either the ND filter factor or the optical density number.
Some cinematographers choose to stack filters to combine the effects of two or more filters. For instance, a cinematographer may use two Neutral Density filters to produce a more powerful reduction in the amount of light that is allowed to enter the camera. For instance, a 6-stop ND filter may be combined with a 10-stop ND filter. In doing so, the cinematographer has created a 16-stop ND filter that will drastically adjust the image that is captured.
Stacking filters is not always the best option as doing so requires the light to pass through multiple layers in which there is always the risk of refraction. When light gets refracted, a softness may occur in the image. This same softness is the complaint that some cinematographers have when using a UV lens for protection of the camera lens.
ND Filter Shapes
ND filters are often round and easy to attach to the front of the camera lens, but not always. Some ND filters are rectangular or square-shaped. These types of filters are inserted into special filter holders in order to produce the desired effects for the lens. Round and rectangular filter ratings are the same, and there are no differences in the filtering quality between the two shapes. The only real difference is how the filter is attached.
Advanced ND Filter Types
In addition to basic ND filters, there is a wide range of ND filters that perform different functions. These include:
● Graduated neutral density filters – GND filters transition from light to dark.
● Variable neutral density filters – VND filters allow the cinematographer to dial-in the amount of light filtration by turning the outer ring of the filter.
● Center neutral density filters – CND filters are dark in the center and have lighter edges to balance exposures.
● Polarizing filters – polarizing ND filters or polarizing filters provide a polarizing effect and are one of the most common ND filters that most cinematographers already own.