Understanding Neutral Density
Q) I’ve noticed a lot of different types of neutral-density filters. Could you describe them and talk about how to use the various kinds?
A) Neutral-density filters lengthen the exposure of the scene without influencing color (hence, the term “neutral.”) The filter artificially darkens the image, and the camera’s sensor lengthens the exposure to compensate for the lack of light. They’re manufactured in a variety of densities (one, two, three and five stops are commercially available) and formats.
A standard neutral-density filter is useful for achieving certain techniques where you want a long exposure in a well-lit area. For example, landscape photographers often use the filter to achieve a sense of movement with water. A polarizing filter can be used to the same effect—it typically loses two stops of light.
A split neutral-density filter, also known as a graduated neutral-density filter, comes in a number of variations. In all of them, the filter is part neutral density and part clear. They come in varying degrees of neutral density that allow the photographer to “hold back” brighter areas of the image so that longer exposures can bring out detail in the darker areas. A typical use is to equalize the exposure of a bright sky and darker foreground in a landscape.
The round, screw-in graduated neutral-density filters aren’t recommended because the line of demarcation between the filtered and unfiltered areas is right in the center of the image, which is rarely the place we want it. The square or rectangular units, with their respective holders, are preferred because the line of demarcation can be moved up and down, or even side to side, and the larger unit can be adapted to fit almost any lens.
A new version of the neutral-density filter is the Vari-ND from Singh-Ray (www.singh-ray.com). This filter uses two highly corrected polarizing filters rotating one in front of the other. As they’re rotated, they offer a varied neutral density of from three to eight stops. The advantage is that the photographer can choose the length of exposure and ƒ-stop by just rotating the two filters until the desired density is achieved.
I used a Singh-Ray Vari-ND to take this vignette of a cascade in Yosemite National Park. A Canon EOS 5D with a 100-400mm lens set to 180mm was used at ƒ/22 and ½ sec.