Calgon, Take Me Away

Here's a slow shutter speed photo taken in the Great Smoky Mountains National Park on the Tennessee side of the park.

I took the photo a couple days ago on a 1900 mile drive from New Mexico to North Carolina. I was picking up a car that my mother had given us. I used the trip as an opportunity to photograph several cities and national and state parks. I'm sure I'll post some more about this trip and the things I learned about planning and executing road trips for photography (a.k.a. the things I screwed up). But for now, let's talk a bit about actual photography.

I've always loved the ethereal look that a slow shutter speed conjures with flowing water. It can transform a pretty but conventional creek into a wood nymphs' magical bathing pool. The technique works wonderfully for rivers, waterfalls, beaches, fountains – wherever you have flowing water creating some turbulence on the surface. And taking these photos is pretty straightforward, yet incredibly satisfying because it's not point-and-shoot instantaneous. There's some planning, set up, and process involved, which makes the final result feel more rewarding. As in, I love it when a plan comes together.

The trick is using a neutral density filter.

ND Filters – The Bare Minimum

Think of ND filters as sunglasses for your camera. They simply reduce the amount of light entering the camera. By reducing the light, a ND filter enables you to use a slow shutter speed to get a correct exposure, and in turn that slow shutter speed creates the characteristic misty, hyper-blur of flowing water.

Here's the same shot without the ND Filter. It's not nearly as cool!

ND filters have two key specifications that you have to get right. First, there's the size. Assuming you're using screw-on filters, your filter size should match the lens you'll be using. With a MILC or DSLR, your lens will usually be labeled with its size in millimeters. You can buy adapters to fit larger filters onto your smaller lenses (going the other direction will result in tunnel vision images). This is very cost-effective because you can buy a filter to fit the largest lens in your collection, then buy relatively cheap adapters to fit the filter on all of your other lenses.

The second key specification is the strength of the filter – i.e. how much light it filters out. Unfortunately, manufacturers use a variety of ways to rate this. Some use ND rating (of which there are multiple variations), some use optical density, some use percentage of light transmitted through the lens, and some use f-stop reduction. All these different ways to measure the same thing over-complicate the purchase of a ND filter because they make comparison shopping difficult. I'm not going to explain the different rating systems in this post because there are already a lot of web sites (including this one) that explain it all and, even better, have tables that convert between rating systems, which you'll need when you start shopping.

For actually taking pictures, however, the filter strength rating you really want is f-stop reduction. This rating measures the filter's light reduction in exposure stops – the same unit of measure used by your camera's exposure controls. The ND filter's f-stop reduction is very useful because if your filter is reducing light by, say, 3 stops, then you simply slow your shutter speed by 3 stops to compensate. Easy peasy.

How strong of a filter do you actually want? For the photo above, I used a 10-stop filter, but you don't necessarily need one that strong. Anywhere from about 4 to 10 stops works for these types of water photos. The stronger the filter, the more misty and dreamy the water will look, but the less motion will be depicted.

Now, I suppose I should mention that there are other features on ND filters that affect durability, image quality, and price. Things like lens coatings, materials, color neutrality, and so on. These things are certainly important, but they won't prevent you from being able to actually take your photo like the filter size and rating will if you get them wrong. Whether or not you opt for these features is a matter of how much you're willing and able to spend to gain improvements in product and image quality.

Ten Easy Pieces

The general workflow is pretty simple:

1. You need a tripod. There's no way to handhold the camera at the shutter speeds this requires. In a pinch, you could set your camera down on the ground or a rock, but the safest (for the camera) and most stable way is a tripod.
2. Settings: Start off in manual or aperture-priority exposure mode. Turn off auto-ISO and set the ISO to the lowest setting, which for my camera is 100.
3. First, without the filter, nail down your composition, depth of field, and focus. Set up your tripod and camera and make sure you have exactly the composition you want. Set your aperture for your desired DoF. For landscape shots, that often means a very small aperture (I used f/11). Acquire focus. Take test images and evaluate them carefully for exposure, focus, DoF, and composition. Repeat until everything is on the money. Getting it right during the test shots will save you a bunch of time later.
4. Once the test image looks right, take note of the aperture and shutter speed settings. Switch to manual exposure mode (if you weren't already there) and replicate the settings.
5. Switch the camera/lens to manual focus mode to lock down the focus. You have to do this because once you put a strong ND filter on your lens, your camera may not get enough light to auto-focus anymore. So you establish focus during your test shots and then lock it down. If you use back button focus like I do, you can skip this step because your focus is already locked down until you press the back button again.
6. Now you can install your filter. Be careful not to change your focus, zoom, or tripod position accidentally. Also, when you're installing the filter, keep one hand underneath the filter to act as a safety net in case you drop it. You don't want it broken, scratched, or lost in the water!
7. Lower the shutter speed by the appropriate number of stops for your ND filter. In other words, for my 10-stop filter, I slowed down the shutter speed by 10 stops. There are a number of free smartphone apps that will do the calculation and tell you exactly what to set the shutter speed to, but I just count clicks while turning my shutter speed dial.
8. If you're using a DSLR, install the eyepiece cap that may have come with your camera. This is really important because at these super-slow shutter speeds, light leaking in from the viewfinder is likely to cause a color cast on your photo. It will look like a purple or brown streak in your image. When that happens, you'll think your ND filter is defective, but the only thing that's defective is the photographer. If you don't have an eyepiece cap, or you lost it, just cover the eyepiece with your hand (being careful not to touch/bump the camera).
9. Take your photo. On this picture, I needed a 90 second shutter speed, which exceeds the longest shutter speed of my camera. I had to use the "TIME" shutter speed setting where you press the shutter button once to open the shutter, then press it again to close it. I used my wrist watch to time the exposure. At those slow shutter speeds, a few seconds plus or minus doesn't have an appreciable effect on the exposure.
10. You may need to adjust the shutter speed a bit from the calculated shutter speed. This is particularly true for really dark filters. For example, with my 10-stop filter the calculated shutter speed resulted in a photo that was a bit underexposed. Providing an additional 1/2 stop of shutter time corrected the exposure.

A Matter Of Time

If you follow these instructions, you can work your way deterministically to a good image every time. You will quickly develop a feel for how long to make your exposures for certain conditions. You can use this technique to capture cloud movement, which is also very cool looking. And let's say you're at a tourist location and there are too many people walking in and through your scene to get a nice clean photo. Using a ND filter and a long shutter speed, the people won't be in one place long enough to register in the exposure and your resulting photo will be the scene without the people!