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What Is A Graduated Filter?
A graduated ND filter, or graduated neutral density filter, is a special type of ND filter. While ND filters consist of a single disk of darkened glass, a graduated ND filter has a gradient on this glass going from dark to light.
The strength of the graduated filter is typically measured by the equivalent number of f-stops that the filter will darken the darkest part of the gradient by. Typically, each stop equates about .3ND, though it varies by the filter manufacturer.
There's a lot of combinations possible here, as you might imagine, so think carefully on your needs. Most photographers tend to prefer a graduated ND filter that darkens by 2 or 3 stops for their first, but again, it depends on the photographer.
Gradients can either be “hard" or soft", that is, with an abrupt transition between light and dark, or a very gradual one. These are useful for different sorts of photographs, and it might be a good idea to have one of each.
The phrase “neutral density" comes from the fact that the filter is absolutely gray, darkening all colors equally. This prevents the image from taking on any particular color cast. You can also use colored graduated filters, which are discussed at the end of the article.
Make sure that you purchase a graduated ND filter where you can adjust where the gradient is located relative to your lens. Without this ability, you'll find your ability to compose images rather stunted. These tend to be the ones that are attached to a rectangular lens attachment, as opposed to the circular ones.
The primary use of graduated ND filters is for shooting with both the sky and the landscape correctly metered:
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Many a sunset is sweet and simple: a flat line between the sky and the ground. Maybe there's a lake or a few trees thrown in, but the idea is the same. It's easy to get the correct exposure on these sorts of photographs—with the aid of a graduated filter. Using one of these filters with a harder, shorter graduation will keep the picture crisp and correctly metered, right along the horizon.
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What if you've got some serious elevation going on here, hills or buildings or some other ups-and-downs that don't make your skyline a flat line? Well, while you can't create a graduated filter that will mold to fit a more complex skyline perfectly, to get as much of the picture as you can correctly meter it's probably better to use a softer, more graduated filter. Also, you'll probably want to err on the side of covering too much of the picture with the dark portion of the filter than too little.
If you're shooting so that the light is coming in from the side, such as for an asymmetrically framed sunset, trying tilting the graduated filter so that the darkest portion is where the image is otherwise the brightest. This may help even out the photograph somewhat.
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Bright Versus Twilight
As for the strength of the graduated filter to use, consider the sky you're looking to darken. About how much brighter is it than the landscape beneath it? It takes a while to get a feel for the exact number of stops you may need, so if you need to experiment a bit, don't fret.
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Of course, graduated filters don't have to be neutrally colored. Ones with a slight color cast can add a lot of flair to a photo, from shades of tobacco brown to warm reds and oranges. This can be used to create a variety of moods in your landscape photos—try them out and experiment!
If you want color but don't want to necessarily invest in a specifically colored graduated filter, then you can also just put a color filter over the top of the normal graduated filter. Custom color filters are very easy to make.
For another article on graduated ND filters with exquisite examples, check out this one from Alex Wise Photography.