In last week’s First-Timer’s Gear Guide, I mentioned that your camera can take hundreds of times longer to properly expose a scene at night than it might during the day time. It’s all too easy to open your shutter long enough that the stars in the scene will start to trail. In this article, I explain how to avoid star trailing when shooting from a fixed tripod.
What Causes Star Trailing?
Don’t get me wrong; I love a good star trails picture, but I can’t stand blurry stars when I’m trying to take a star field photo or a sharp view of the Milky Way. So let’s try to understand what’s going on.
Figure 1: 100% crops of a 120-second exposure (left) and the same region again at 20 seconds (right). Taken from a fixed tripod with a Canon EOS 6D and EF 16-35mm F/2.8L lens at 16mm.
As the Earth orbits the Sun, it also rotates about its axis once per day. Our brains are tuned to perceive visual changes many times per second, so we don’t even notice the earth’s rotation when we look up at the stars. But cameras need more time to capture photons than the human eye, so long that the stars in the scene move across portions of the camera’s sensor during capture. This is what causes star trailing in our night sky images.
Think of it like being on a merry-go-round. When the merry-go-round spins slowly enough, you still see the playground in sharp detail. When the merry-go-round spins quickly, the environment around you starts to blur. Your eyes can’t capture the scene quickly enough for your brain to render it in sharp detail. The same thing can happen to your camera when you take long exposures of the night sky.
The “500 Rule”
Perhaps the simplest solution to star trailing is known as the “500 Rule”. The 500 rule isn’t perfect, and I’ll explain why in a moment, but it produces reasonably good maximum exposure times based on the focal length of your lens. It is best-described by the following equation.
is the shutter time in seconds
is the focal length of the lens, in millimeters
is the crop factor, relative to Full Frame sensor size. This means if you’re using a Full Frame camera (~35mm sensor), you can ignore this term. Canon’s APS-C crop sensors usually have a factor of 1.6 while Nikon crop sensors have a crop factor of 1.5, but you will want to check to be sure. If no crop factor is published for your camera, you can calculate it yourself by dividing 35mm by the length of the diagonal of your camera’s sensor.
A drawback of the 500 Rule is that it completely ignores a couple of important factors. The first factor ignored by the 500 Rule is that the speed at which a star appears to move in the night sky is dependent upon how close it is to the nearest Pole (north or south). Take this photo, for instance.
Figure 2: Star trails over Mount Taylor, NM (click here to view in gallery). This photo represents about 3 hours of total exposure time. Taken with a Canon EOS 6D and a Rokinon 12mm F/2.8 fisheye lens.
This photo shows about 3 hours of the Earth’s motion. Notice how the stars near Polaris don’t move very far in the image, but the stars closer to the Celestial Equator (the imaginary plane exactly halfway between the North and South Poles) move quite a bit. In other words, if I’m taking a picture with the north pole in it, I can get away with a longer exposure time than I can if I’m pointing my camera at the Celestial Equator. The 500 Rule doesn’t take this into account.
The second issue is that, while the 500 Rule takes into account the overall size of your camera’s sensor, it ignores how big or small the pixels are. Think about it this way. If your camera sensor has very large pixels, it will take a lot longer for a star to move from one pixel to the next than if your camera has very small pixels. For instance, the Canon 6D and Canon 5Ds have basically the same overall sensor size, but the 5Ds has 2.5x as many pixels. When measured by surface area, each pixel in the 5Ds is less than half the size of each pixel in the 6D! So star trailing may be much more apparent in 100% crops taken from a 5Ds than it will be from a 6D when using identical lenses and exposure times.
The 500 Rule is handy for figuring out general exposure times. I use it all the time. That said, you shouldn’t just blindly trust it. The first time you take a lens out to shoot the stars, use the 500 Rule to get an initial estimate, shoot near the Celestial Equator, then experiment until you find the exposure time you think is optimal for that particular lens/body combo. The good thing is, once you’ve found that optimal exposure time for your lens/body combo, it will never change, so you can use it again and again.
Another thing to consider is that a little star trailing in your images is okay, depending on the resolution you expect to display the image at. If you just want to make a nice wallpaper for your phone, star trailing of several pixels or will be imperceptible. If you want to print a massive poster of the same image, you might be less amenable to the same amount of star trailing. It all depends on what you want to do.