Photographing Star Trails

Introduction

Star Field
(Death Valley, California, USA)

Star Trails
(Death Valley, California, USA)

When photographing stars, you can either get a star "field," a static snapshot of the stars as points of light, or star "trails," where the stars' movements streak across the sky. How long you expose the image determines which you get. The first rule of thumb to remember is that the Earth rotates such that the light from a star begins to "move" after about 30-40 seconds. (It's apparent movement is largely dependent on your lens—the longer the focal length, the more apparent the movement; the wider angle lenses won't show much movement till later because of the star point is so small.) Part of your experimentation will be to gauge the timing for how much "trail" you want.

My very first truly successful star trails picture: a 3.5 hour shot on a night with no moon, 16,000 feet above Mt. Kilimanjaro
(Kilimanjaro, Tanzania)

Photographing star trails is technically simple; the main things to keep in mind are:

Timing the light

Compositing the scene

Watching battery power

Experimenting a lot

Most people who get started with shooting star trails want to capture the longest trails they can by keeping the shutter open longer. The inherent problem with this is underestimating the ambient light in the sky, even though you don't necessarily see it. This light may come from nearby cities, or even the diminishing sunset an hour or more afterwards. What your eye sees is nothing compared to a long exposure of a camera, where this residual light can be so overwhelming, you don't see any stars at all. A 20 minute exposure an hour after sunset can look like a day shot, and if the moon is anything more than crescent, you'll be limited to just a few minutes at best. (By comparison, a full moon will make a night shot look like a day shot in about 8-10 minutes at f2.8 at ISO 100.)

Picking a faraway place on a night with a new moon (or where the moon hasn't yet risen, or after it's set) is best for getting the darkest skies, which make the light from the stars is more pronounced. This may not be as easy as you think. The photograph of the lit tents shown here was shot in Death Valley (over 300 miles away from Las Vegas), which still had an illuminating effect on the horizon. To illuminate the tent, I spent about 30 seconds waving a flashlight around from inside the tent. This process is hard to get right without overexposing the tent's fabric. Again, the benefits of experimentation. This light also helped bring out detail on the ground.

Composition

Using a flashlight, wave it around inside a tent for about 30 seconds. (20-minute exposure.)
(Death Valley, California, USA)

While photos of nothing but star fields and trails are fascinating and will impress your friends and neighbors, they can get pretty old pretty fast if that's the only thing in your photo. These pictures are much better with foreground subjects. Think daytime photography here: a lake, an interesting tree, rock formation, or even your house. As you experiment with various shots, the first thing that'll pop out at you as you see the actual photos, is the direction of the star trails themselves. This is never apparent when shooting the picture because you don't actually see the stars move at all. Hence, the direction of where the stars move will become an increasingly important element in choosing your compositions.

On a perfectly dark nite under a totally new moon, you won't have much to see other than stars. With no other light at all—even the ambient light—you can't see other objects at all in the foreground, which makes for limited composition options. Here, it's common to get a silhouette of a tree or a mountain. Although it's also fun to use a flashlight to illuminate foreground subjects like a cactus. (I've even used the brake lights and turn signals from my car to create colorful red and yellow bursting effects on foreground objects.) Starting an exposure while a crescent moon is just about to set can illuminate the foreground enough to have them lit adequately, while permitting the exposure to continue (and get longer trails) as it gets darker.

Two Hour Exposure out a hotel room window
(Alto Adige, Italy)

Most people use the Polaris, the northern hemisphere pole start as a point of reference for composition decisions. (Those who live in the southern part of the planet can use the southern pole star.) As the Earth rotates, stars will appear to spin around it, as shown in the photo of Gusela Mountain, in the Italian Dolomites.

In choosing foreground subjects, you'll need to make sure you get them in focus along with the distant stars, which often requires smaller apertures. This works against you because the smaller aperture means less light. Alas, your composition has trade-offs: composing a scene that can uses a wide aperture, but still has interesting foreground subjects. Generally, I compose scenes that have the closest subject to the lens be at least 10 feet in front of me, if not further. I also use a wide angle lens, so that I can get everything in focus at or f3.5 up to f5.6. I try to avoid apertures greater than f8, which can yield a reasonable picture if exposed for several hours or more. Again, I don't want to raise my ISO setting, because the digital noise becomes intolerable. However, doing so is a good way to experiment, and get a prototype of what your composition will look like for a longer exposure at a lower ISO.

Exposure Times

One reason why long exposures are hard: there are many things that can ruin your picture... like a late-night car driving where it shouldn't be.
(Logarska Dolina, Slovenia)

Because of the different sources of light and the great effects such subtle changes can have on a very long exposure, you can't really "calculate exposure times" here. The camera's light meter is irrelevant, especially for exposures that are going to be well into the minutes, if not hours. This requires setting the shooting mode to "manual" or "bulb" and using a cable release. Some advanced cable releases have timers built into them, whereas manual versions require you to push the cable yourself. If you think that's not so bad, keep in mind that you're going to have to be around (and awake) in several hours when you want to release that button. Every camera manufacturer has different cable releases to choose from, but after having done this for a while, I can speak confidently that having one with a timer is well-worth the money.

At this juncture, you must now experiment and rely on trial and error to learn the ropes. Just compose what you might think would work, release the shutter, and go get coffee (or go to sleep).

It's tempting to want to use a higher ISO setting to brighten photos due to the lower light conditions, but the side-effect of higher ISO is higher noise. I use ISO 100 to keep the "digital noise" down, which is more pronounced in darker areas of an image than lighter ones. Your mileage may vary, as different camera manufacturers deal with noise differently. Even though some cameras may perform better than others, all cameras will produce much better images at lower ISOs than higher ones.

Powering the Camera

3 Hour Exposure Facing East (North Star is upper-left, southern point is below horizon on bottom-right.)
(Morocco)

Almost any film-based SLR will expose long enough because battery consumption is low. But digital cameras have a tougher time because digital sensors eat power. So much so, that most cameras won't do much more than 30-40 minutes on a single charge, regardless of the battery type or camera model. It therefore becomes part of the evening's landscape choice to find a location where you can use your power cable. When staying in hotels, I often choose rooms that face towards the darkest part of the sky and that have the least amount of ambient light (usually the decorative hotel lights). This way, I plug into the wall, and place the camera (on tripod) either on the balcony, or shoot through the window.

When camping, I use an AC power adaptor plugged into the car or a nearby power supply. (A house, cabin, etc.) Obviously, this may not always be possible. In this case, true night-photo nuts go out and buy generators or battery packs that can keep a good exposure going for quite some time.

To get more images in less time—and to make the most of battery power if you're not using a power adaptor—opt for simpler night pictures of shorter star trails, or just star fields. As stationary objects, it's still a pretty amazing site.

Fogging Lenses

During a full moon, exposures can only last about 8 minutes before over-exposing.
(Patagonia)

A common problem with night photography is dew fogging the glass because ambient air is warmer than the lens itself. What causes dew isn't the temperature, it's the amount of humidity in the air (though cold fronts tend to have drier air). Metal lenses will always be colder than the ambient temperature, but even lenses with plastic barrels can suffer from the problem simply because of the glass components inside the lens. The solution is keep the lens warmer than the air. How you do that is the challenge. The most fool-proof way is to get a bulky battery pack and wire it up to your lens. Places that sell astronomy equipment make these for larger telescopes, but they are too large for normal camera equipment—it'd be like shooting a mouse with an elephant gun. The solution "works" and won't damage anything, but it's an overkill.

A more common and inexpensive way to prevent humidity from building up on the glass is to use one of those hand-warmers that you put inside gloves or boots for winter activities, like skiing. One particular brand is at www.warmers.com. They cost about $1-2 each, and while that site sells in bulk, you can buy them one or two at a time at most outdoor outfitters like www.rei.com. Wrap one or two around your lens with a rubber band, and it'll last anywhere from 4-8 hours. Of course, if it's really humid, like at the top of mountains in Peru, where clouds hover around your tent, it's darn near impossible to avoid the dew.

Light trails on the ground were from the flashlights pointing down used by people walking by. (Yosemite, California, USA)

20-minute exposure of star trails, with trees lit by faint ambient light from the campfire (Yosemite, California, USA)

More to See

For a complete list of all star trails shots on this site, go here.