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What Is A Neutral Density Filter?
ND filters derive their name from the neutral effect they have on color while still darkening the image: absolutely gray, dimming all colors equally. This may seem simple, but it's a powerful tool. You've probably run across scenes that are just too bright to photograph the way you want to. This is where ND filters come in handy.
There are a three main varieties of ND filters: solid ND filters, ND filter wheels, and graduated ND filters. Solid ND filters are the most common out of the three, consisting of a single filter of even darkness. ND filter wheels are made with two glass disks that each have a gradient of increasing darkness which can be rotated relative to each other, allowing a whole range of light and dark to be easily manipulated by the photographer. Graduated ND filters are basically ND filters with a single gradient from darker to lighter. Use of these are covered in a different article.
ND filters tend to be classified by the “f-stop reduction", that is, measuring how much darker the image is made by how much darker it would have otherwise been made by increasing the f-stop by that much. Another system of classification is by the percent of the light that passes through the filter, or transmittance.
Not all ND filters are created equal. Even the best of them will have a slight color cast. Make sure that you check out the quality of the ND filter carefully before purchase.
Don't have an ND filter? Check out this article for how to make a DIY ND filter.
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Aperture & Exposure
ND filters essentially allow you to have greater manual control, especially over your aperture (f-stop) and exposure, while in bright environments. With the entire scene essentially set to a darker bottom line, one can bring the aperture lower, allowing for a wider variety of focusing effects, or set the exposure longer, allowing for motion blurs and other cool tricks.
There are a variety of classic examples where ND filters come into common use:
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Sometimes, no matter how short your exposure or high your f-stop, there are still those nasty overexposed areas cropping up in your image because it's just flat out too bright. An ND filter can get that baseline brightness low enough that you don't have to necessarily use the highest f-stop and shortest exposure your camera can manage.
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Water is constantly moving before our eyes, but how to capture this in a single moment on the camera? But not capturing a single moment, but several of them, exposing for a few seconds. This will turn the water into a shimmering blur, creating the illusion of movement to our mind. An ND filter allows you to set the longer exposures necessary even with bright, sunny days where normally you'd be setting your exposure at hundredths or thousandths of a second.
So, think up anywhere you might be seeing moving water: waterfalls, seasides, river, any place that might benefit from a little artistically blurred water.
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Artistic motion blurs can be easily created as well with the aid of a ND filter. This can be especially handy for surrealistic effects with the motion of people, dancing, talking, walking, in otherwise normal lighting situations. Mixtures of blurred and unblurred portions of the body can be especially intriguing, a still face while the body sways, or the batting of eyes or the swinging of feet.
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Fun With Focus
On bright days, it may often be difficult to drop the exposure down low enough while also keeping your f-stop low enough that you can create the effect that you want—especially for those indistinguishably blurred backgrounds that many macro artists love, or so that a particular feature of the landscape sticks out slightly more than the rest.
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Hot Hot Hot!
While these situations aren't quite as common, some photographers will find themselves trying to photograph objects that are extremely hot—and quite bright. This can be anywhere from a glassblower's studio to the depths of a fire—just use your imagination. Keep in mind, however, that most ND filters do not block UV or IR wavelengths, so if that becomes an issue, then you'll have to use the additional filters.