Tip Of The Week: Use ND Filters to Blur MotionTips
Every week, we post a photography-related tip on our blog. These tips are typically inspired by questions we get from our customers. Sometimes we might feature a technique tip, and sometimes a gear recommendation. If there’s something specific you’d like to see in this section, let us know. Email us at [email protected]
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, filters 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 quite a bit.
As is wont to happen, what’s old is slowly becoming new again. Of late, there’s been a resurgence in the use of certain filters, to the point where Schneider, one of the leading companies that makes these filters, is back-ordered on a number of them.
What is an ND filter, and why do I want to use one?
But let’s backtrack and talk about what an ND filter is, exactly.
In simple terms, an ND – or Neutral Density – 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).
Now, why would you use an ND filter? Well, there are a number of reasons for that. Here’s an excerpt from a previous post…
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 24fps) or 1/60th of a second (if you’re shooting at 30fps). The reason behind this goes back to the old film days and the way the 180-degree rotating shutter worked on old film cameras. Tyler Ginter has an excellent post detailing how this works on his blog, if you’re interested.
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, since the low-light performance of bodies like the Canon 1D Mark IV are quite amazing.
But when it comes down to a question of too much light (say, at high noon, for example), videographers often have a problem. You can shut down your aperture, of course, and pull your ISO down to 100 (or even 50), but at some point, you hit a wall and can’t go any lower on ISO or smaller on your aperture. Also, sometimes you don’t want that small aperture and its deep depth of field. Sometimes you want a really shallow depth of field at high noon.
Also, sometimes you don’t want that small aperture and its deep depth of field. Sometimes you want a really shallow depth of field at high noon. There’s a good video showing this principle in practice here.
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, when you want to keep your aperture wide open, but can’t go any higher on shutter speed or lower with your ISO.
There’s also another very cool side-effect of using ND filters: you can use it 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 2 of the San Francisco skyline, shot from Treasure Island, shows a really choppy sea, which isn’t very pretty. Using two stacked ND filters similar to the Schneider NDs we rent, I was able to increase my shutter speed to make a 30-second-long exposure and blur out the choppy bay. With a crop and a few edits in Lightroom and Photoshop, I got the image you see in Figure 3.
The filters I use (which include a 10-stop ND) and the Schneider filters we rent require a holder or a mattebox to that places them securely in front of your lens (watch for more info on the Schneider filters in next week’s Tip of the Week), but you can also use a circular filter like the Singh-Ray Variable ND filter we rent as well.
A similar method was used in Figure 4. Using a two-stop ND filter to expose the Legion of Honor museum in San Francisco late in the evening, 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. So, I stuck a 2-stop ND filter in front of my camera, increased the exposure to 60 seconds, and viola!
In the space of the two 60-second shots required to make this stitched image, four cars crossed in front of me, but the only thing they left behind were streaks of light. A simple layer mask in Photoshop blended the two images enough so that the break in light streaks between the two images fades nicely.
That’s how useful this little piece of dark glass is. In the shot of the the San Francisco Skyline, by the way, a small boat speeding right-to-left near the left edge of my image is not visible, because of that same principle.
Despite the time and patience it takes to set up and experiment with ND filters, the results you can get with them are really cool. Without them, some images are simply not possible, or at least, 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 done in Photoshop. The ocean chop, for example, could be blurred in post, but the one produced by using the two stacked ND filters are much, much more organic-looking.
So pick up some dark glass, and get shooting!