Photography Tip: Neutral Density (ND) Filters and Long Exposures

Neutral Density (ND) Filters

Oftentimes, photographers love bright light. Bright light allows us to shorten our shutter speed and/or shoot with decreased aperture (higher “f” number, like f/11 or f/16 compared to f/2). More of the scene can be in focus because of a wider depth of field, and we can do things like freeze water droplets or birds in flight. However, there are certain times when we don’t want that much light reaching our camera’s sensor. Intentionally reducing light also allows for creative photography. Lengthening the shutter speed allows us to get silky water effects or blurry skies. Shooting with the aperture wide open, for example at f/2, can give beautiful bokeh in bright light.

Enter the neutral density filter. A neutral density filter reduces the amount of light entering your lens. The strength of the filters is typically given as a rating number, which can be read as the denominator in a fraction. For example, a ND 8x filter reduces the amount of light to 1/8th, whereas a ND 400x filter reduces the amount of light entering your lens to 1/400th compared to if no ND filter is used.

Reasons to Use ND Filters

There are a few reasons to use ND filters. One example is when there is simply too much light. If you’re shooting in broad daylight with the sun directed towards your camera, you might be forced to use an extremely fast shutter speed. Maybe faster than your camera can handle to get a decently exposed image.

Another example is for water shots. If you’ve been following my blog, you know that I LOVE shooting water. That silky smooth water effect can be achieved by slowing down the shutter speed. Depending on how turbulent the water is, you might not need to slow down the shutter speed by much. But if the water is slowly moving, you might need to shoot for upwards of 20 seconds to get that effect. Longer shutter speeds means more light hitting the sensor, and ND filters can counter that. Sometimes I even stack ND filters to further reduce the amount of light. A tripod is often necessary for these shots.

Here is an example from Lake Kaweah, California. In the first image, I did not use a ND filter. I shot this in aperture priority mode, with a shutter speed of 1/20sec.

Here’s the same scene with a 20sec shutter speed after attaching a ND x400 filter.

Here’s another example from Vernal Fall at Yosemite National Park. The runoff was pretty rapid, so I didn’t have to slow down the shutter speed too much to get the silky effect.

One of the other really cool things about slowing down the shutter speed is that you can get rid of things moving across the scene. The following example is from Donut Falls, Utah. There is a makeshift “bridge” that people use to cross the water. I wanted to get a shot of the flowing water without the people, but there was so much foot traffic that waiting for an opening wasn’t feasible. So I set up my tripod, stacked both my ND x8 and ND x400 filters, and adjusted my exposure compensation to +2.0EV to slow the shutter speed even more, and took the shot. The two images below were shot back-to-back, and you can see that the people disappear from the image on the right because of the slower shutter speed.

Another interesting thing you can do with water and ND filters is get an opaque glassy effect. This works in closed bodies of water with a little bit of turbulence, like a lake. In the following image, from Lake Kaweah, California, take a look at the smoothness of the water. You can see the rocks and foliage underneath the water, and the water almost looks like stained glass.

Some Things to Consider

There are a few things to consider if you decide to slap on ND filters to take multi-second photographs:

1. You may have noticed that some of the images above have foliage in the foreground. The longer shutter speeds might blur the foliage if it’s windy. Because of this, it might be a good idea to frame up your shot without things that can move in the foreground, unless you don’t mind it. Alternatively, you can clone them out in post-processing.
2. Slower shutter speeds means you should have some way to stabilize the camera to reduce camera shake. A sturdy tripod goes a long way. Sometimes I’ve had to improvise and used a towel on a rock when I didn’t have my tripod!
3. Putting any form of glass, such as filters, in front of your lens can affect image quality. Cheaper filters might diffract light before hitting the sensor. Some filters can also cause vignetting and/or chromatic aberration. The more you stack filters, the higher the chance this could happen.

If you have any questions, or have some additional tips to share, please feel free to use the comment box below!