ND Filters - What You Need to Know

While ND filters are extensively used in the film industry to control exposure where shutter speeds are limited, many photographers are unsure whether they should purchase one for their camera lens. The main function of a ND filter is to allow a photographer to use wide apertures in bright lighting conditions. In other words, an ND filter is like “sunglasses” for your lens. When you put an ND filter on your lens, the scene is not as bright. There are several scenarios where this function can be applied in photography.

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Neutral density filters are grey-toned and in theory should be neutral, meaning there is no color shift to cool or warm and no tint of green or purple. The darkened glass is designed to absorb light as it passes into the lens and onto the camera sensor. ND filters come in various strengths which are measured as stops of light ranging from ND2 (1 stop) to ND1000 (10 stops) and beyond. Each “stop” of light blocking prevents 50% (half) of the light from coming into the camera. ND filter strengths are indicated in four possible ways – ND factor, stops, ND1 numbers, or optical density. There is a chart at the bottom of this article that shows these different notations.* Just be aware of how ND filtering is measured when you make a purchase to be sure you get the strength level you wanted.

Robert Emperley from Strasbourg, Alsace, France, CC BY-SA 2.0, via Wikimedia Commons A demonstration of the effect of a neutral density filter. Note that the photograph was exposed for the view through the filter, and thus the remainder of the scene is overexposed. If the exposure had instead been set for the unfiltered background, it would appear properly exposed while the view through the filter would be dark.

Some ND filters offer only one consistent strength level and some offer variable strengths that are adjusted by rotating the filter. Whereas a standard ND filter has an even density from edge to edge, a graduated ND filter is clear on one side and gradually builds up density (a gradient) toward the other side. Graduated ND filters are almost exclusively used by landscape photographers to even out scenes with exposure extremes, such as a bright sky with a shaded or darker valley and foreground. Graduated ND filters are typically sold as square drop in filters so you can properly line up the horizon. Standard ND filters may be sold as square drop in filters or circular ones that screw onto the end of your lens. A variable ND will be circular so that the two pieces of glass can be rotated to get the appropriate strength effect.

ND filters are mainly used for portraits and landscape photography. They make it possible to use extended shutter speeds to “smooth” the look of flowing water, clouds, or traffic and blur movement. In bright lighting conditions an ND filter (depending on the strength) allows a photographer to use fully wide open apertures for shallow depth of field.  Let’s look at some specific scenarios to learn how an ND filter might benefit your photography.

If you want to take long exposures during the daytime, a strong ND filter is a necessity in your camera bag. Long exposures are used to get motion blur in water, clouds, traffic, crowds of people, and more. In the example photo below, an ND filter allowed the shutter to stay open a much longer time (long exposure) and as a result, the moving water becomes a soft, white blur. Without the ND, a long exposure would create an extremely overexposed or all-white image. When used properly, the photographer still gets the same overall brightness that they would have gotten with proper exposure without the filter. As an example, if the scene below required settings of ISO 100, f/8, and 1/500th with no filter in place, then we can adjust to a slower shutter speed and add the appropriate level of ND strength to the lens to create the same exposure and the result is the same level of overall brightness. If the shutter had to be adjusted to 2 seconds to create this amount of blur, that means we changed the shutter speed by 10 stops (1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, ¼, ½, 1 second, 2 seconds). We would need a 10 stop ND filter to compensate. You can learn to do these calculations yourself, or there are apps for long exposure that can calculate for you.

Adrian Kirby - https://pixabay.com/photos/seljalandsfoss-waterfall-iceland-1207958/

An ND filter can be beneficial for portrait photographers. Typically a lower strength ND filter would be used for portraits along with a prime lens at a wide aperture for blur or shallow depth of field. Imagine you want to shoot at f/1.2 or f/1.4 and it is very sunny outside. The aperture is wide open. You already adjusted the ISO as low as it will go (ISO 50 or 100) and you’ve got the fastest shutter speed you can get from your camera (1/4000 or 1/8000) and the image is still too bright. To bring down the exposure and keep the shallow depth of field, you need an ND filter for that portrait. Without an ND filter your only option would be to close down the aperture to f/2.8 or maybe more.

Image via Pixabay. If your image is too bright and you want to keep your aperture wide, an ND filter can help.

Another use for ND filters and portraiture is to block light so that you can use a strobe or flash at the regular sync speed (if you don’t have high speed sync or don’t want to use high speed sync). The sync speed of a camera and flash is the maximum shutter speed where the camera’s computer can match the timing of the flash with the opening of the shutter and still use the maximum power of the flash. High speed sync will allow you to use a faster shutter, but you lose some of the strength of the flash. Therefore, if you want to use the full power of your flash, for example during outdoor portraits on a sunny day, the normal synced shutter speed may be too slow and result in overexposure by letting in too much light. Adding an ND filter to your lens can correct the exposure.

The final scenario we will discuss in this article is the use of a graduated ND filter for landscapes. A graduated filter is darker at the top, gets lighter in the middle and is clear at the bottom. If you need to take a photo of a landscape that has a dark foreground, but the sky is bright, then you position the center of the filter over the horizon line (where the dark filtering is fading). This will even out the exposure. It is not a magic fix-all, but it will keep the sky from being blown out or the foreground from looking like a silhouette, and it will let you get the image “in camera” instead of blending bracketed exposures in post/editing. The filter is just subduing parts of the scene enough to even out the exposure. If you use an ND filter that is too strong, your sky might have rich colors, but then anything that crosses the horizon into the sky (such as trees) will be unnaturally dark. You don’t want an abrupt transition and you don’t want to overdo the level or strength of the ND you choose to use. You will still need to even everything out in processing.

BenFrantzDale~commonswiki assumed (based on copyright claims)., CC BY-SA 3.0 , via Wikimedia Commons -- A GND filter held up to the horizon. Note the poor contrast in the overexposed part of the sky not covered by the filter and notice how bottom of the filter is clear. Exposure of that part of the water is the same in areas with and without the filter.

As with any filter, adding additional glass elements to the lens can cause glare, ghosting, and flares. These can usually be removed in editing. Changing your position or using a lens hood can help eliminate these effects. Stacking an ND filter with other filters, such as a polarizer or UV filter, is not recommended because it can create more or worsen these effects. A filter with multi-coatings can also help prevents these effects.

Slim filters lack threads on front side, therefore the slimmer profile is less likely to create a vignette (darkening in the corners). Slim filters are a good option for wide angle lenses and zoom lenses.

More expensive filters will be more neutral. Cheaper filters may have a color shift. The quality is usually better when you spend a bit more. Get filters with metal rings and glass, not plastic! Carefully read the reviews and look at sample images before you buy and be careful buying online. Many filters sold online are knock-offs. The best place to source your filters is direct from the manufacturer or an authorized retailer like B&H Photo, Adorama, or Best Buy.

If you purchase a variable ND, you can save some money because this one filter will offer many different strength levels instead of buying individual ND filters for each level or strength. I like to buy my filters in a larger size and use step down rings to fit them on multiple lenses instead of buying filters for each individual lens. Variable ND filters can also be helpful when the light is changing fast as they are easier to quickly adjust to keep the same level of exposure. The main drawback to purchasing a variable ND is the dreaded “X” or cross pattern that can be created as you rotate the filters. Since the filter blocks light from certain angles using two pieces of glass, the light that is blocked isn’t always uniform. You can sometimes see artifacts, patchy or uneven cross-like patterns or a vignette that gets worse with the higher ND filtration. To mitigate this issue, try to use the variable ND below its maximum.

Image taken with K&F Concept ND2-400 Slim Variable filter set at about 8 stops. When viewed at 100% there are small artifacts visible and slight vignetting in the corners (most easily noticed in top left corner where the sky is darker blue). This was not a high resolution image - taken on an 8MP old Canon Rebel with 17-40mm lens at 17mm. Using the slim filter and keeping it set below its maximum strength will help eliminate unwanted effects. This K&F filter advertises it has no dreaded "X" and in my practice this has been true.

Don’t purchase a graduated ND unless you plan to take landscape photos frequently and will need to darken skies during sunrise, sunset, or to balance out really bright skies. You don’t need a graduated ND taking up space in your bag if you only take landscape photos occasionally. The work around is to take a bracketed exposure of three images (one photo exposed for the sky or background, one photo exposed for the mid-ground, and one photo exposed for the foreground) which you then blend in post.

Keep in mind that there are different types of graduated ND filters, such as hard (sharp line between dark and light areas) and soft (more gradual transition). The soft graduated ND filters are often easier to use, especially for beginners, and a better option when there are objects along the horizon line (trees, buildings).

So, in summary, ND filters can be a good addition to the camera bag, primarily for portrait and landscape photographers. They can be a bit tricky to use, but understanding how they work and the shooting scenarios they are intended to correct will help you get better images overall.

ND Filters are available from ND1 to ND100000 and beyond. An ND100000 is the minimum strength needed for photographing the sun.

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