Use Neutral Density Filters to Blur Motion

The use of various filters – physical ones, not the ones in Photoshop – is something that waxes and wanes with time. Back in the film days, they were an indispensable part of the landscape photographer’s toolkit. With the advent of digital photography and technologies like HDR, the use of filters (especially graduated and colored filters) has fallen off. What’s old is becoming new again. There’s been a resurgence in the use of certain filters. Let’s talk about Neutral Density Filters.

What Are Neutral Density Filters?

In simple terms, a Neutral Density (ND) filter is a dark piece of glass or resin that cuts down the amount of light coming into your camera. It does so without, hopefully, affecting the white balance of your image or adding a color cast. Though, as you’ll see later, this isn’t always the case.

Filters and Exposure

One thing that confuses a lot of photographers is that in the video world, shutter speed is no longer something you can use to control your exposure. At least, not without additional consequences. When shooting video, your shutter speed needs to be fixed at either 1/50th of a second (if you’re shooting video at 24 FPS) or 1/60th of a second (if you’re shooting at 30 FPS). The reason behind this goes back to the old film days and the way the 180º rotating shutter worked on old film cameras. Learn more about this in An Introduction to Frame Rates, Video Resolutions, and the Rolling Shutter Effect. Bottom line: you have to shoot at a fixed shutter speed, which means you have to control exposure through the use of your aperture and ISO settings. For lower-light situations, this is ideal; you can shoot with your shutter wide-open and keep jacking your ISO up to get the exposure you want. The low light performance of modern cameras is quite amazing.

But when it comes to too much light (like at high noon), videographers have a problem. You can close down your aperture, of course. You can pull your ISO down to 100 (or even 50). But at some point you hit a wall. You can’t go any lower on ISO or smaller on your aperture. Sometimes you don’t want that small of an aperture and its deep depth of field. Sometimes you want a really shallow depth of field at high noon. The same exact thing applies to still photographers. ND filters can be used to control your depth of field while shooting in really bright situations. They are handy when you want to keep your aperture wide open but can’t go any higher on shutter speed or lower with your ISO.

Figure 1: A choppy bay in the foreground of the San Francisco skyline.

Figure 2. The bay was smoothed out with the use of ND filters.

Filters and Motion

There’s also another very cool side-effect of using ND filters. You can them to blur motion and make moving objects disappear completely. If an object doesn’t stay still in your frame long enough to be properly recorded on the sensor, it’ll either show up as a blur, or, if it’s fast enough, won’t show up at all. The image shown in Figure 1 of the San Francisco skyline shows a choppy sea. It isn’t very pretty. Using two stacked ND filters, I was able to increase my shutter speed to make a 30 second exposure. This blurred out the choppy bay (Figure 2).

The filters I use (which include a 10-stop ND) require a holder or a matte box. This holds the glass sheet in front of my lens, like this Schneider ND 4×5.65 Filter Set. But you can also use a circular, screw-on filter like this variable ND filter for rent. A similar method was used in Figure 3. I used a 2-stop ND filter to expose the Legion of Honor museum in San Francisco. I was able to bypass an annoying problem. My initial exposure was for 15 seconds. But cars were still passing by in front of the museum at that hour – blurry but very noticeable. I stuck a 2-stop ND filter in front of my lens and increased the exposure to 60 seconds. This kept the exposure levels I wanted but made the cars nearly disappear into pleasing streaks of light.

Figure 3: ND filters were used to blur out the vehicles passing in front of the shot here.

That’s how useful this little piece of dark glass is. In Figure 2 of the the San Francisco skyline, a small boat was speeding right-to-left near the edge of my image. But it is not visible because I was able to use a very long exposure even in daylight.

Conclusion

Despite the time and patience it takes to set up and experiment with ND filters, the results you get with them are really cool. Without them, some images are simply not possible. Or, at least, they would need a ton of work in Photoshop to pull off. The blurs and fades produced by these filters are more natural than most things I’ve seen in Photoshop. The ocean chop, for example, could be blurred in post. But the one produced by using the two stacked ND filters is much more organic-looking. So pick up some dark glass and get shooting!