There are essentially two ways to shoot star trails. You can lock the camera shutter open for an extended amount of time (typically upwards of 10 minutes), or you can shoot a series of 30-second exposures and use a simple piece of software to stack them together. I prefer the latter strategy, as if something happens during one or two of the exposures — such as someone shining a torch at your camera — you can simply discard any affected shots, whereas with a single, lengthy exposure if anything like this happens then the entire exposure is ruined.
Before stacking the images in post-production, there are a few things to consider when shooting. You can only shoot star trails when the skies are clear and light pollution is limited. Even a small cloud passing through the scene will create a white smudge that can be difficult to edit out. Set up your camera on a good tripod for accuracy when stacking and crop so that there is something interesting in the foreground, and a lot of sky in the picture.
Try using a relatively wide aperture and shallow depth of field to show up the greatest number of stars. Shooting is essentially trial and error, and you will probably have to use a relatively high ISO, but rest assured that any noise will be reduced by the stacking process. After taking the series of shots, the final step — without moving the camera — is to refocus on the foreground, and then reduce the ISO to as low as possible and shoot a single high-quality image with an extended shutter speed. For this shot you will need to switch on the Long Exposure Noise Reduction.
Step 1: Lightroom
Heading back in to the warm, the next step is to assemble all of the images together. Depending on how long, and how late you have been shooting, this might be a job for breakfast.
Import all of the images into Adobe Lightroom, and post-process the first of the series of shots: adjusting the basic tools such as white balance, exposure, white point, black point and shadows. You should also switch on Enable Profile Corrections in the Lens Corrections palette, to reduce any distortions.
Once you are happy with the image, hold down the shift key and click on the last image in the series and the entire set of images will be selected. Click on the Sync button and then apply all of your corrections to the series of images. Export all of the images in TIFF format to a folder on your computer and then export the single image that you have shot for the foreground somewhere else. You will be using this later.
To assemble the images I use a small (and free) program called StarStaX (starstax.net). This has OSX, Linux and Windows versions. Select the folder of processed images and then, under the Preferences panel, select Blending, and then Gap Filling as the blending mode. This will automatically retouch the tiny gaps between each exposure. Then, under the edit menu, simply select Start Processing and sit back and watch your Star Trails shot assemble.
Step 2: Photoshop
Focusing on the foreground
With the Magic Wand Tool (W), draw a rough selection in the sky, which will now become selected. Select > Inverse to select the non-sky elements of the picture.
Click on the Quick Mask icon. This is towards the bottom of the toolbox and is a rectangle with a dotted circle in the middle. The non-selected areas of the image will now be masked with a red overlay. Using the paintbrush tool to add to the mask, and the eraser to delete from it, edit the mask so that it just covers the foreground areas.
When you are happy with the mask, click on the Click Mask icon again: the areas covered by the mask will now be selected with an animated dotted line.
Open up the high-quality image of the foreground, selecting Edit > Select All, and the Edit > Copy. Return to the original document which will still have the selection active, and then go to Edit > Paste Special > Paste Into. This will paste the foreground document into the selected area and, as long as you haven’t moved the camera during shooting, it should fit perfectly.
If you need to make any local edits to hide the join of the pasted-in section, then this can be done with the Clone Stamp Tool.
Published in issue 6 of National Geographic Traveller (UK) Photography Magazine
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