The Epic Battle Between Choosing Star Trails Over Star Points
One of the main reasons why photographers lug their cameras around when the sun goes down, is to capture the night sky. For a lot of us, this means leaving the bright lights of the city in search of where the stars shine the brightest. Until recently, the only way a photographer could successfully capture the night sky was with long exposures that resulted in star trails. If you wanted to capture star points, or a more celestial night, exposures needed to be less than 30 seconds. Otherwise, the earth’s rotation turned the points into trails. But with DSLRs now capable of capturing cleaner ISO output at 1600 and 3200, we are entering a new celestial era that would make Van Gogh proud!
So whether you choose the drama of the longer star trails or the subtler star points, here are some tips to follow.
Star trails begin to take shape when we make an exposure for over two minutes. The key to star trails is the direction in which you point your camera. Remember—it is the Earth’s rotation that is causing the stars to trail. In the Northern Hemisphere, Polaris, or the North Star, is in alignment with the northern polar axis of the Earth.
What does this mean to the photographer?
The North Star will remain constant during long exposures, and all the other stars will appear to revolve around it. The further you move your camera away from the North Star, the longer the trails will appear in the image.
How do you identify the North Star? Look due north using a compass or a phone with a compass app. It will be one of the brightest stars due north. It is one of the easiest stars to find.
As the world turns—and we lengthen our exposures—we will create longer star trails. The closer the star trail is to the North Star, the shorter and more curved it will be. If you capture the eastern sky, the trails will look like a curved /, while the west will look more like a slightly curved \. The farther you move from the North Star, the longer and straighter the trails become. Due south will be the longest, and relatively straight.
Thus a 6-minute exposure due south will be longer and straighter than a 6-minute exposure to the north. The way that the star trails curve (or do not curve) can play a big part in your image. When I took the image of the “light house” (above) and star trails, I could have shot it at several different angles. But I wanted the trails to seem like bolts of light coming out of the tower. I knew that if I pointed my camera south the trails wouldn’t bend, and I held also my exposure to about 4 minutes, to lessen any curving.
The image of Bannerman Castle was taken with a film camera for over 1 1/2 hours, with a wide angle lens that can capture a 93º angle. It is a perfect example of dramatic long star trails. It includes the North Star, as well as the longer trails to the west.
My preference is to shoot under a full moon, because it will help light up any foreground information—the castle, for instance. However, you have to be aware that the moon and city lights will sometimes outshine stars in their vicinity. If you can’t see the stars, your camera can’t either. Be aware of clouds as well; they can break up the trails and turn your night sky to mush.
A perfect night to capture that celestial sky would be a moonless, clear night away from city lights.
But first you have to ask yourself: How does your DSLR handle those higher ISOs of 1600-6400? That is one area that camera companies are constantly improving, and if capturing this type of image is important, I would use the most recent model. But test your higher ISOs, and see if they can be reasonably tamed in post.
The main rule to capturing star points is the 600 rule:
If your DSLR is a full frame, 35mm format, divide 600 by the focal length of your lens, and the result represents your maximum exposure time, in seconds, before star points start to trail.
For example, my preferred night lens is the Zeiss 21mm f2.8.
The math would be 600/21 = 28.57 seconds.
A 50mm lens would be only 12 seconds.
So it is more beneficial to shoot with wider-angle lenses when trying to capture star points. The 21mm lens lets in over one more stop of light when compared to the 50mm—which means not having to raise your ISOs. Plus, the wider the lens, the more stars you’ll include.
If you have an APS-C-sized sensor, you divide 600 by the crop factor of the camera to figure out your new starting point. Nikon and Sony are 1.5x, and Canon is 1.6x, so it would be 400 or 375 respectively. Then do the same math. For example a Nikon D7000 with a 17-55 f2.8 lens at 17mm would be 400/17= 23.5 seconds.
So you can see the difference a full frame with a wide-angle lens makes.
Once you have the maximum time that you can expose before stars trail, you then play the game of ISO and f-stops. I don’t like to shoot wide open, so typically I will do a test shot of stopping my aperture down to f/4, and setting my ISO for either 6400 or 12,800.
Hopefully, we can bring the ISO down to 3200 or 1600, because that makes the noise easier to control when processing them later.
Foreground is a consideration as well. The shot on the left has the Valley of Fire way in the distance and impossible to “light up.” But I feel the elephant’s-eye silhouette plays nicely against the starry night. The shot on the right was taken from a scenic viewpoint along the highway in New Hampshire, and I had the help of a passing car’s front light help me light up the wall or granite perfectly. Got to love free light painting!
You can see more of Gabe's work, and his night adventures, on his blog: Ruinism. If you're interested, you can also check out his workshop with famed photographer Tim Cooper over at Rocky Mountain School of Photography, called "Vegas to Zion: Dusk to Dawn."