Capturing Star Trails
We do everything possible to avoid star trails when shooting the Milky Way, but sometimes the trails are exactly what we aim to capture. Long exposures of star trails create gorgeous images of the night sky that depict the passage of time.
In the northern hemisphere, all of the stars in the night sky appear to revolve around the North Star, or Polaris. If you shoot a long exposure with your camera pointed at Polaris, you’ll notice that all of the stars circle around it. Use Polaris as your guide when choosing a direction to shoot. The two outermost stars in the bowl of the Big Dipper point to Polaris.
You have two options when shooting long exposures of star trails. You can either take a single long-exposure photograph using a very low ISO, or you can take a sequence of images that can be stacked in software like StarStax. For example, instead of a single 60-minute exposure, you can shoot 120 images of 30-second exposures taken consecutively.
Stacking exposures is the recommended method. First of all, the longer your exposure is, the more your camera sensor heats and generates noise in your image. Secondly, the stacking method makes it possible to photograph star trails in light-polluted areas. Even if you’re in a bright city, you could shoot 1,000 images of two or three-second exposures and still capture star trails. Finally, many different factors could ruin a single long-exposure, such as a dead battery, tripod shake, or an unexpected light source entering the frame.
While a wide-angle lens is best for the Milky Way, you can use a lens of any focal length to photograph star trails. Note that the wider the lens is, the longer you’ll have to wait for star trails to fill your frame. If you remember the 500 Rule example from before, you need to wait around 35 seconds for star trails to appear with a 14mm lens on a full-frame camera. Using an 85mm lens instead, you only need to wait 500/85, or about 5 seconds.
Using a longer focal length is your best choice if you want longer star trails to appear over a shorter period of time. Use the widest aperture setting on your lens so you can shoot at the lowest possible ISO when exposing your image. Always remember that a higher ISO means more noise.
Place your camera in continuous shooting mode and set the shutter speed to your desired exposure time. By locking the shutter button down on your shutter-release cable, your camera will take consecutive images as each exposure ends until you stop it. Alternatively, you can use the built-in intervalometer on your camera (if it has one) to set the desired parameters.
Don’t forget to bring extra batteries! Long exposures and cold nights will quickly drain their life.
Photographing the night sky is a blank canvas of possibilities for a photographer. It will take a lot of experimentation to discover which settings work best for you. In the process, you’ll gain a deeper understanding and fascination for the cosmos.