First off, this is not intended to be a comprehensive tutorial, but simply a list of the steps (with some brief notes) that I took to generate this image.
My first experience with star trails was in junior high using film. In many respects, using film simplified the entire process; one simply opened the shutter, walked away, and returned a few hours later. Most importantly, however, was the fact that regardless of the duration of the exposure – whether 10 minutes or 4 hours – the noise was the same. With digital cameras noise becomes a serious challenge to overcome. That’s why in most instances, stacking many shorter exposures (with low noise) is preferred to taking one long exposure. (There is also a practical upside to this approach. For example, In the event that a source of light pollution makes its way into your composition, you can simply remove those exposures from the final stack.)
Here then are the basic steps, from pre-visualization through post-processing, I took to get this image:
- Scout the location
- Ideally you want a prominent and interesting foreground element. Taking an image of a portion of the sky provides no context for the composition.
- You need to find a location, or be facing in a direction, that minimizes light pollution (easier said than done). This image was taken in a rural area of New York. Even still, light pollution from towns miles away can cause problems. Here, the hills in the foreground not only provide a nice silhouette, but they also keep light pollution on the horizon from interfering with the composition.
- Use planetarium software to check the night sky
- Though not necessary, it’s often aesthetically pleasing to have Polaris (the North Star) in, or near, your composition to see the concentric rings traced out by the stars. You absolutely must be prepared with information about the moon, specifically, when and where it will be in the night sky and how much of it will be illuminated. A bright moon can make capturing all but the brightest star trails very difficult. That’s not to say that if the moon is out there are no opportunities for night photography. In fact, the moon provides a soft, even source of illumination that can be used to great creative effect. This was taken the night of September 16, a new moon, which is ideal for capturing as many star trails as possible.
- Check the weather
- This should go without saying. Pay special attention to cloud cover, temperature, and humidity. The evening of this shot it was cool (50’s) and perfectly clear.
Taking the Shot
When out in the field, be sure to start with a fully charged battery, particularly if you are shooting in cold temperatures. Remove any filters from your lens which may attenuate the light (i.e. circular polarizers). In addition to your camera, you’ll need a sturdy tripod and an intervalometer. If you’re new to shooting in low light situations, I highly recommend reading Floris Van Breugal’s tutorial on The Twilight Hour.
- Capture foreground shots at twilight
- Getting this image was a long term commitment to a single composition. I began taking the first images shortly after the sun set to get a nice, color gradient in the sky to use as the “canvas” on which to place the star trails. I then waited over an hour before taking the star trails (which themselves lasted an hour).
- If necessary, use an external light source (headlamp, flashlight, flash, or moon) to “light paint” and bring out details and textures in your foreground. In my final image I blended 3 different foreground shots to get the desired effect. In two of the images a friend light painted the tree with a headlamp. In the third, a large billboard next the road behind my setup was reflecting light on the field which illuminated the grass.
- Take star trails (e.g. “light frames”)
- Taking the actual light frames involves a good understanding of your camera’s ISO performance and your tolerance for noise in a finished image. I shoot with a Canon 5D Mark II and found an in-depth analysis on its ISO performance which guided my choice of settings (ISO 2000).
- Equally important is understanding the performance of your lens. Ideally you want to shoot with a lens that allows you capture as much of the night sky (wide angle) and collect as much of the light (large aperture) as possible. I used a Canon EF 16-35 mm f/2.8L lens at f/2.8 and 16 mm. The lens certainly has shortcomings at these settings, namely, lack of sharpness near the edges. However I was willing to sacrifice sharpness (they’re just streaks of light after all) for the ability to collect more photons.
- Using the information about how your camera deals with noise, it’s performance at various ISO settings and the performance of your lens will inform your choice of exposure time. I used 45 seconds.
- Take dark frames
- Digital sensors create a dark signal that depends on the exposure time, ISO setting and temperature. To record the dark signal – so we can later subtract it out – simply take an image with the lens cap on, making sure to use the same camera settings as your light images. Since the dark signal is temperature dependent, take the dark frames in the field immediately before, or after, the light frames.
- Take bias frames
- Each digital sensor creates a readout signal when data is read from the chip. To record the readout signal (once again, so we can subtract it out later) simply take an image with the lens cap on, using the fastest shutter speed your camera allows. The readout signal is not affected by temperature but is dependent on ISO, so use the same value as for the light frames.
- Use Adobe Lightroom to batch process light images. For each:
- Apply “Lens Correction” to correct barrel distortion and vignetting.
- Adjust “White Balance” to more closely match twilight conditions (or to your taste).
- Reduce the exposure by 1-stop, set “Tone Curve” to linear, but make no other changes. The reason for reducing the exposure by a stop will soon become apparent.
- Process Dark and Bias Frames (Optional)
At this point, the process begins to take a turn towards astrophotography. I doubt that using dark and bias frames were necessary for my relatively short exposures (45 seconds). Yet, given how infrequently I have the opportunity to indulge in night photography, I wanted the best chance possible of capturing a high quality (i.e. noise free) image. While these steps may not be necessary for star trail images, they certainly are relevant for static shots of the night sky.
- Apply “Lens Correction” to dark and bias frames
- In Photoshop create a master bias frame by averaging your frames to reduce noise
- For each dark frame, subtract the master bias frame
- Create a master dark frame by averaging your frames in the same way as Step (2)
- For each light image, subtract the master bias and dark frames
- Stack light images
If you’ve ever experimented with stacking images to generate star trails, you’ve probably seen the result where trails exhibit small gaps. While you may think this is due to the time between exposures, it’s actually caused by the blending mode. To avoid these artifacts in the final image requires using the Lighten-Screen-Blend (LSB) method, whereby a combination of the “Lighten” and “Screen” blend modes are used to stack the images. Though straightforward in principle, the LSB method can be tedious and time consuming when dealing with more than a handful of images. Fortunately there is a free script available to do the work for you.
- Use the script to stack all your light images. Note: the resulting image will look very flat and have little contrast, all things that will be remedied in Photoshop.
- Blend foreground and star trails
- First, make adjustments to your stacked star trails image. Typically you need to adjust contrast, levels (or curves). I’ve also found Nik’s Tonal Contrast filter to be indispensable, particularly adjusting the Shadow Contrast. This allows fainter star trails to really “pop”, however you need to exercise caution as it also introduces noise into the image.
- The final step is to layer the completed star trail image with your foreground image. Experiment with blend modes and masks to get the right combination of foreground and star trails.This step can be nontrivial – I spent hours and hours finding the right balance.
When I envisioned this shot, I wanted to create the comet-like effect in the star trails – and I expected to do that in post-processing. During the course of the hour long worth of exposures, however, condensation began to accumulated on the front lens element. I thought my shot was beyond repair. But in a happy coincidence, what actually happened was, the condensation gradually attenuated the light, creating the very effect I wanted in (and literally, on) camera!
Finally, let’s go through some numbers. The final image required the use of:
- 80 light frames
- 10 dark frames
- 20 bias frames
- 3 frames blended for the foreground
That’s a total of 113 images with a cumulative exposure time of little under 70 minutes. Further, at 60.1 MB per TIFF file, the final image is comprised of (113 x 60.1 MB) roughly 6.8 GB worth of data. Whew!
I hope this has been helpful. Feel free to contact me with any comments, questions or suggestions!