Startrail images can be shot with either a single long exposure or by stacking multiple shorter exposures. The two methods can give very different results, and each method has it’s own advantages and disadvantages.
One of the biggest advantages of shooting a single long exposure is the reduced processing time compared to stacking. You can load the image into PS/LR and jump straight to the final processing stage, avoiding the time consuming steps of cleaning/stacking/blending multiple exposures.
When shooting a single exposure, the exposure settings will be determined by the amount of light pollution in the shot. The image above is a good example of what happens when you don’t nail the settings. The light pollution on the horizon is overexposed to the point that it can’t be saved in post.
My normal process for a shot like this would be to shoot test images at short shutter speed and high iso to get the settings dialed in, then extend the shutter speed and reduce the iso/aperture to compensate. (I didn’t use this test method for the image above, I just took a guess).
For example, if my test shot was 120 seconds at iso1000, I could change the settings to 1200 seconds at iso100 for longer trails, or 600 seconds at iso200 which would capture more stars but the trails would only be half as long. The overall exposure would be the same for all 3 images.
A single exposure can be ruined by things like a distant car passing through the shot, or a wayward flashlight. The light trails from the car in the image below aren’t too bad and could be removed. If this happened in a stacked image the first one would be a very easy fix, the second one would take a bit longer but wouldn’t be that hard (keep in mind that in a stacked image the trails from a single exposure would normally be a bit shorter than the trails in this image).
There are other issues that can arise, for example dew forming on the lens towards the end of the shoot, or clouds moving in during the shoot. Both of these can ruin a single exposure image but if it happens with a stacked sequence you could just discard those images.
Stacking Multiple Exposures
Stacking is my preferred method, as it allows me to capture lots of stars and have very long trails, and I have a lot more control over the exposure of all the elements in the image such as stars, empty areas of the sky, landscape and light pollution. If I encounter any of the issues mentioned in the single exposure section above they can be dealt with relatively easily. The two images below were included in the stacked image above. That’s a phone and a flashlight that found their way into the frame.
The downside to this method is the time spent in post. The image above took about 5 hours to edit (cleaning plane trails, stacking stars, blending twilight foreground shot and final editing), and I have a very fast PC. If you’re doing one of these on a laptop with a high MP camera it’s a good idea to clear your schedule for a week.
Stacking also gives you the option of tapering off the ends of the trails using the ‘comet fade’ effect, or blending in a twilight image for better lighting on the landscape parts of the image.
There’s also the option of doing something in between the two methods, stacking a small number of longer exposures to give the look of the single exposure method but with longer trails.