Have you ever looked at an image of the night sky where the stars leave long, arching trails? These images, called “star trails”, record the movement of the stars as the earth spins around its axis. This is a compelling way for you to capture a phenomenon not observable to the human eye.
As with many creative photography techniques, there are can be a steep learning curve for shooting star trails. A basic understanding of the night sky, knowing the impact of focal length, and composition can help you maximize a night out with the goal of shooting star trails.
Some fairly simple post-processing techniques can help you perfect your shot once home. I will walk you through the basic technique of photographing star trails, help you think about how your gear affects the outcome, and highlight two post-processing techniques that I find useful.
1 – Basic Technique for Shooting Star Trails
The concept of the shooting star trails is actually pretty simple. Set up your camera on a tripod and then compose the shot. I recommend disabling autofocus at this time. Once you have your composure right, set the exposure on your camera. In general, you will want to expose the shot for as long as possible without over-exposure. You can check the histogram of your image to decide if it has been overexposed or not.
On dark nights without a moon or light pollution, you can start in Manual Mode, ISO 800, f/2.8 (or lower/wider), and 15-seconds. Modify these settings to best capture your scene. Once you are happy with the exposure, you will need to set your camera to take pictures at a steady interval. You can do this by setting your camera’s internal interval meter or by attaching an external intervalometer.
Each camera model has different intervalometer connections or internal settings (or may not have this feature), so consult your camera’s manual to get this set up correctly. As you set up your camera, think about the number of shots you want to take. The more shots you take the more the stars will move – with many lenses you will start to capture significant movement in about 8 minutes.
Below I will go through a couple scenarios where fewer or more shots may be better. As a rule of thumb, I shoot for a minimum of 45 minutes and as long as several hours. Let your camera shoot and enjoy the night sky!
Processing or Stacking the Star Trails
Once I am done with the shoot, I import the photos to Lightroom and Photoshop (using Adobe Bridge). There are other star-stacking programs that you can experiment with, but I like Photoshop for this task. To import the photos from Adobe Bridge open the program and then navigate to where the photos are stored. Highlight the photos you want to include in the star trail and then go to Tools –> Photoshop –> Load Layers into Photoshop As Layers.
Once the layers have loaded into Photoshop you may want to use the Auto-align feature (disregard this if you are certain your tripod did not shift) by highlighting the layers in the right panel and then going to Edit –> Auto-Align Layers –> Auto. The final step is the use the “lighten” blend mode in the Layers panel and apply it to all the layers.
The lighten function examines all of the overlapping layers and then keeps only the lightest pixel. Keep that in mind as you can use it to your advantage (examples of that below). To complete the image export it to a lossless format (I like TIFF). You may then continue to edit the new TIFF in Lightroom or Photoshop.
2 – Shooting Tips
Know the North Star
Having a basic understanding of astronomy will aid you as you compose your shot. The North Star is often the focus of star trails because it does not move in the sky as the earth spins on its axis. To find it, locate Ursa Major (e.g., The Big Dipper) and then follow the line created by the stars at the end of the dipper to locate the North Star.
Star apps on your phone are also a great way to locate the dipper or the North Star. Once you know where it is, you can use it in your shot. I often like to bury the North Star behind a piece of a foreground element giving the final image a pinwheel effect.
3 – Find Some Foreground Elements
Foreground elements are always important for landscape photography. When shooting star trails, think about foreground elements that capture the essence of the scene or that you can place prominently and by themselves.
By this, I mean objects that stand away from the background of the image. You may also want to choose elements that can be lit by the light of the moon or by using light painting. I like to think of star trails as telling the story of the night and the objects that you include in that frame will aid you in that storytelling.
4 – Use Light Painting
Using Adobe Photoshop’s Lighten blend mode provides a lot of options for creativity when shooting star trails! Remember, that Lighten only keeps the lightest pixels in the whole stack of images. So by using light painting, you can selectively lighten objects in the frame.
Illuminate the foreground with your phone, headlamp, or another light source. You can light up the whole thing or selectively light elements of it. Experiment with lighting angles, intensities, and colors. If you don’t like the lighting of a certain exposure simply remove it from the layers that you import into Photoshop. I usually spend the first 10-20 frames lighting the foreground to make sure that I capture the lighting that I want and then let the camera take the rest of the shots.
5 – Pick Your Lens Focal Length Wisely
Focal length will strongly impact the amount of time it takes for the stars to move in your shot. Shorter focal lengths (e.g, 14mm) will take longer for the stars to have trails than longer focal lengths (e.g., 50mm). Knowing this will help you plan your shot. The three images below emphasize this effect.
MThe wide-angle of my 14mm wide-angle lens allowed me to capture the North Star and a distant mountain landscape, but I stood there for three hours to get the amount of movement in the stars that I wanted. The second shot was taken at 50mm and only 45-minutes elapsed before significant movement in the stars occurred. The third shot is an extreme example, shot at 300mm. The green streak is Comet Lovejoy and shows only a couple minutes of movement.
6 – Play with Exposure Times
The length of your exposure will strongly influence the final image that you create. There are no guidelines to what is the right length, instead, you should be guided by what looks good to your eye. As a tip – you can always choose to use fewer shots than you captured so by default I would take as many images as you think you’ll need and then modify the amount once you import them into Photoshop.
Which of the images below do you like better? The longer exposure or the shorter one?
7 – Compositing Tips
Photoshop gives you a lot of flexibility to mask and preserve or remove elements of the shot. Since star trail shots are composite images and thus art, I do not worry about these alterations from an ethical standpoint. The two techniques below may help you improve the final shot. They assume you are familiar with masking and healing in Adobe Photoshop. If not you will find the linked articles helpful!
Masking a Foreground
Once you compile the images you may find elements that draw your eye away from the phenomenon you are trying to capture. Since the horizon and foreground are the same for all of the images you can choose which foreground looks best to you. Use that foreground to create a selection and convert the selection to a mask.
You can use that mask and exported TIFF file to maintain the foreground you like. In the images below, I wanted to remove the hikers that walked up to the lava flows in Volcanoes National Park and the bright highlights of the lava which became overexposed as the lava moved. I used a mask to preserve the foreground elements I liked.
Editing out Planes and Satellites
Almost all dark nights will have a plane or a satellite come through your frame. Fortunately, these are very easy to remove! Use the Healing Brush tool and set the tool to replace and content aware. You can draw a linear line with the tool over the track of the satellite or plane. Voila! The offending track will disappear.
So there you go! I hope this article can help you get out there on your first night of shooting star trails. Remember, knowing your stars, picking a foreground and playing with exposure length will help bring the shot you imagine to reality.
Once you process the stacked images you have lots of flexibility in Photoshop to fix parts you do not like. As I always say, pixels are cheap. So make lots of them as you learn to shoot and process star trails.