There are two filters that almost any professional photographer always has in his or her gear bag: a neutral density (ND) filter and a graduated neutral density (GND) filter.
These two filters are useful because they allow you to control exposure. By using these, you can reduce the amount of light reaching your camera’s image sensor either over the entire scene of your image (in the case of the standard ND filter) or over just a portion it (as with a GND filter). This reduction in light allows for leeway in terms of the shutter speed and aperture settings that are required for proper exposure of an image and this, in turn, gives the photographer the opportunity to use these settings more creatively.
So let’s talk about the basics of neutral density filters.
Neutral Density Filters
A neutral density (ND) filter reduces the amount of light that enters the camera lens. This type of filter is neutral gray in color so that the amount of light entering the lens is reduced, but the color of the light isn’t changed. The darker the gray color, the more the light is reduced.
Reducing the light entering your lens gives you added flexibility in terms of exposure. A neutral density filter can allow you to use a large aperture opening to create a shallow depth of field, or a slow shutter speed to create the blur of motion, even when the actual scene lighting won’t allow for these exposure settings. By using a neutral density filter, you make the scene darker so that the slow shutter speed or the wide-open aperture work to give your image proper exposure.
ND filters comes in varying strengths. And, not surprising, different manufacturers use different systems for specifying the strength of their filters. There are several different methods for labeling neutral density filters.
But, first, in order to understand these different labeling methods, you need to be familiar with the term stop. In photography, light and exposure settings are discussed in terms of stops. A stop represents a relative change in the brightness of light. If you increase light by one stop, you’re doubling it. Conversely, if you decrease light by one stop, the amount of light is halved.
Okay, so with that in mind, let’s talk about how neutral density filters are labeled and what that label means relative to how much light the filter blocks.
Some filters are labeled by the number of stops the filter blocks.
So under this labeling method, a filter would be labeled as one stop, two stops, three stops, etc. One stop means half the light is filtered out; two stops cuts the light in half again, to give just a fourth of the light that would otherwise be coming through the lens.