Ever since the first time I looked in a book with long-exposure photography, I’ve loved the look of trailing stars in photographs. The idea is simple, expose for a long enough period of time to record the rotation of the stars as the Earth spins. My own attempts were never very successful using film due to the negative effects of reciprocity and my lack of technical skills and patience. Even later, when I finally got a digital SLR camera with the battery life to pull off an hour long exposure, my star streak photos were always sub par, riddled with technical and aesthetic problems. One of my problems was that I always tried to shoot star streaks as if I were shooting a film camera: One very long, guessed exposure, teetering on the edge of my battery life. As I will outline, there are many problems with this technique and there is a much better solution!
The “Traditional” Method of Getting Star Streak Photographs
Things used to seem quite simple. Find a dark sky without too much light pollution, calculate an exposure based on the ISO, working aperture and phase of the moon, place your camera on “bulb” mode and expose for a long enough period of time so that the stars streak through the sky as the Earth rotates. As simple as this seems, there are many problems with this method. For one, while digital cameras don’t have the issue of reciprocity like film does, they do have the inherent problem of noise. Without getting too technical, all digital cameras use an internal imaging device to convert light into an electric charge, this is either a CCD or a CMOS in most cases. As the camera exposes, and depending on the ambient temperature, the sensor begins to heat up and produce thermal noise or dark current. This can heavily degrade the detail and quality of an image. Most modern digital SLRs have built in long exposure noise reduction by silently taking second “dark frame” for the same length of the first, thus removing the dark current. One of the major problems with the built in noise reduction is that in order to be effective, the camera has to noise reduce under the same ambient temperature as the original exposure(meaning the photographer has to stay on location for twice the amount of time), the other problem is battery life limitations. With my older Nikon D200 + battery grip, I could pull off about an hour of exposure + an hour of noise reduction and just barely have juice to preview the final shot.
Another problem with the so called “Traditional” method is balancing a light polluted sky with the foreground. Unless you are just going for a shot of the stars in the sky and don’t care about any foreground detail, you are most likely going to be very frustrated when you find that the sky is too bright to make out the stars due to light pollution if exposing for foreground detail (see the photo on the left). City lights, car headlights and especially a large phase moon can light up the sky dramatically brighter than the foreground leaving you with the choice of either underexposure your foreground, or losing precious star detail and contrast by exposing for foreground detail. Painting the foreground with light is one solution, but doesn’t solve the other problems.
So instead of shooting a single long exposure, we can take a large number of short exposures equaling the length of time that our single long exposure would have been and “stack” them together. Let me clarify. Lets say you would like to have an hour long exposure of the stars moving across the sky. You could set up and shoot an hour long single exposure and run in to the previously mentioned problems, or instead you could do the following…
- You’ll need a fully charged battery (battery grip is recommended), a cable release or remote, a tripod and a memory card capable of holding a few hundred shots.
- Set your quality to .jpg-fine or RAW (Keep in mind that if you want to shoot raw, you will have to batch process a large number of files).
- On a tripod, compose your shot and set your shutter speed to the slowest speed your camera will shoot continuously, most digital SLRs will shoot as slow as 30 seconds, don’t use bulb.
- Taking a few test exposures, set your exposure (using your ISO and aperture) so that you can see stars in the sky. Don’t overexpose the sky, we want a fairly dark histogram, with a very small spike near the right for the stars. Also, don’t worry about getting too much foreground detail in your exposure right now, just focus on the stars.
- Set your white-balance for the sky. This is a matter of personal preference and it will vary depending on the light pollution.
- Turn off any kind of long-exposure noise reduction that is built in to your camera. High ISO NR can be left on.
- Set your camera to continuous drive mode.
- Lock your cable release/remote so that the camera is stuck firing continuously. Now the camera will shoot one 30 second exposure after another until you stop it.
- Stop after an hour (or however long your predetermined exposure is). The longer you let the camera shoot, the more the earth will rotate and the longer the star streaks. Also, keep an eye on the camera as some cameras do not have the buffer to handle so many shots in a row.
- Don’t move the camera! Once you’re done exposing, you still need to shoot a manual dark frame for noise reduction. Put your lens cap on, make sure the viewfinder doesn’t have any light shining into it and expose for 30 seconds.
- The last thing you need to do it shoot a brighter foreground frame for the detail in the foreground. Put your camera on bulb, and make an exposure long enough to get good detail in the foreground, don’t worry about overexposing the sky. I recommend using the same aperture and ISO setting as the 30 second exposures for noise consistency. You can enable in camera long exposure noise reduction for this if you want, or you can shoot another dark frame with the lens cap on of the same length for noise reduction. I recommend turning NR back on for this if you have the time/battery life.
- Go home and follow the next section for processing.
Note that Windows users can also follow the OSX instructions for a bit more work, but also a little more control.
- Download StarTrails from HERE. We will also be using Adobe Photoshop CS3 (although you can probably use CS2 or even Elements 6).
- Open StarTrails and go to File –> “Open Images” and select all of the frames except for the dark frame and foreground frame.
- Go to File —> “Open Dark Frames” and select your darkframe
- Click Build —> Star Trails and give it a few minutes. You can watch as the star streaks grow.
- Save as a .tiff file and open the .tiff in Photoshop
- Also open the foreground exposure jpeg and paste it as a new layer on top of your stacked .tiff file in photoshop
- Create a black mask on the foreground layer and paint in the foreground carefully (see graphic above for example).
- Apply curves, noise reduction and other editing adjustments and your done!
Since the StarTrails Software doesn’t work in OSX, we can use Photoshop to do the same thing. Its a bit more work, but it gives you more individual frame control. There are two ways to do it:
Method 1: Use the CS3 script under File —> Scripts —> “Load Files into Stack.” Open all of your exposures except the foreground frame. Change each layer’s blending type to “lighten” and the dark frame’s to “difference.” This is time consuming and tedious. I recommend method 2.
Method 2: Download this Photoshop action (this does everything above automatically, except the dark and foreground frames). Follow the directions on the site (one step). I recommend deleting the action step where it flattens the layers. Then add your dark frame as a new layer, change the blend type to difference. Next add your foreground image as a layer, create a black mask and paint in the foreground. Apply curves, noise reduction and other adjustments as needed.
Almost Perfect? What gives?
While the stacking technique gives you a ton of control, it is definitely more work. Also, at large print sizes, small gaps can appear in the star streaks from the small gaps between exposures.
- You can include the polar axis (rotational center) in your shot by including Polaris (the North star) in your photo (see the Tahoe photo at the top of this post)
- Shoot during a new moon or smaller than half phase if possible
- Look up local moon rise/moon set times so the rising moon doesn’t mess you up in the middle of an exposure
- If you have a large budget, look at battery grips and/or external power sources for your camera such as Quantum’s Turbo 2×2 battery
If you took a bunch of stacking photos, you can always put them together as a time lapse like this:
That’s it for now, feel free to comment or contact me with any questions.