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Sunday, May 1, 2005

Gadget-Bag: Graduated NDs In A Digital World

These essential filters bring exposure for sky and ground inside a manageable range

gadget bagWhen I was a photography student shooting a sunset 20 years ago, I encountered a vexing problem. When I metered for the sky glowing above Death Valley's mountains in California, I got a wildly different reading than I did when I metered off the dunes below the horizon. Because I knew my slide film couldn't handle more than a three- or four-stop exposure range, I realized I couldn't get detail in both the sunset and the landscape beneath it. Sure enough, the shots exposed for the sky showed a silhouetted landform, and the shots exposed for the dunes blew out the bright sky completely.

The challenge still exists today, but there's an effective solution: graduated neutral-density filters. Also known as grad NDs, split NDs or just plain grads, these filters have a neutral-density coating over half of their area and are clear on the other half. The graduated boundary between light and dark helps make the transition between the two areas indistinct so that your image will look natural.

Placed in front of your lens, they hold back exposure from lighter areas, like the sky in my sunset, and allow you to increase exposure in the image's darker areas. The result is an image with midtones and plenty of detail in what would have been a dark foreground, and sky tones that also are correctly exposed.

Like polarizing filters, grad NDs have an important place in digital photography. Although a lot of people think that Photoshop can fix everything, or that shooting RAW gives you limitless dynamic range, it's simply not true. Like film, most imaging sensors can't capture clean detail in both the highlights and deep shadow areas when contrast is high. If you try to push the limits, you'll either blow out your highlights (they can't be put back) or get a lot of noise in your shadows. Ultimately, even a RAW file won't provide image detail that your camera's sensor is unable to capture in the first place.

You also can use a grad ND when an extreme range of light and dark tones isn't a problem. Say you've got a nice shot set up, but the sky is a bit pale or bland. A one-stop grad ND filter artfully placed over a part of that sky can deepen the color and make your photo look more dramatic. If you want to add a splash of color, graduated filters are available in a variety of hues. They can turn a gray sky into something more interesting or help make up for a lackluster sunset.



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