Single, Stacked, and Tracked for Aurora - OH MY!
Even the word has a mythical and enchanting aura surrounding it. In Latin, the word Aurora means "dawn". Which makes sense, as these lights - if strong enough - can mimic the first light of dawn. At least to the naked eye. The celestial light show captured the imaginations of humans throughout history, and sparked many legends and myths. This allure carries on today, without a doubt!
Today we have amazing technology that allows us to capture this beautiful phenomena. Digital camera technology has progressed an an astounding rate over the past decade. When you think about it, the idea of capturing the Milky Way without extremely expensive and fancy gear is relativity new. Virtually ANY camera - even your cell phone - can capture the Aurora.
Now, that's not to say that the lights were never photographed before - they were. Film cameras were absolutely used and capable of capturing the Aurora. The more powerful displays are actually quite bright. However, in the age of digital, we have more latitude to experiment and play around without the costs involved of developing film.
That brings me to the central theme of this blog post. How do you photograph the Northern Lights? Is there a sure-fire "right" way? In short, no.
Over the past two years, I've had the opportunity to photograph the lights on several occasions. Every time I try or do something slightly different. I have run the gamut in terms of night photography techniques - single images, stacked images, tracked images, stacked and tracked images, ect. This is what I've found - ultimately it comes down to your personal preference and tolerance for noise.
Before we dive into the nuts and bolts of what the different techniques are, lets discuss shutter speed. With the Aurora, this is crucial. Your entire image will look different based on this setting alone. Why? The Aurora is moving. But so is the Milky Way and stars right? That's correct, but from our frame of reference, the motion of the far away objects is infinitesimally small.
Because I suck at explaining some of these highly mathematically and technical subjects, I'll let Robert Frost - an instructor and flight controller at NASA do it.
Q. Why do stars always appear in the same Constellations?
A. From Robert Frost - "Imagine you're looking across a field at one of your friends walking. Behind them you see an airplane flying in the same direction. Your friend and the airplane appear to be moving at about the same speed. But you know that isn't true. Your friend is walking at 4 mph (6.5 km/h) and the airplane is flying at 600 mph (965 km/h).
They appear to be moving at the same speed because they are moving at about the same angular speed - meaning they cover about the same angle in the same amount of time, even though the airplane is covering much more distance in that time.
The farther away an object is, the greater distance it needs to travel to cover the same angle of the sky.
So, let's look at an example involving a star. Here's a picture of the big dipper. The highlighted star is named Dubhe. It is 124 light years away. That's 728,933,976,576,791.5 miles (1,173,105,521,600,000 km) away.
The angular distance between Dubhe and the star below it (Merak) is 5 degrees. Dubhe is moving at 26.5 km/s relative to our solar system. Let's pretend that the velocity vector is pointed towards Merak. How long will it take until Dubhe is overlapping Merak (assuming Merak is not moving, for simplicity)?
A little trigonometry can help us figure this out.
The opposite side equals 102,633,434,259,666 km. That's the distance Dubhe would have to travel to appear to be in the same place in the sky as Merak.
At 26.5 km/s, it would take Dubhe 122,727 years to cover that distance.
So, although all of the stars we gaze upon are moving in their various directions, they are so far away that their angular speed appears to be infinitesimally small. For them to move far enough to be tangibly noticeable to us, would take far longer than we've been observing the skies."
Long story short - the lights are closer to us and moving at a different rate than the stars. Shutter speed will make a difference! But don't confuse this motion with the other motion that effects night photography - the Earth's rotation!
Ok, back on topic. How does shutter speed effect your Aurora images? The answer lies in the basic principles of shutter speed with daylight photography. The shorter the shutter, the more you will "freeze" the motion. Want more defined pillars? Use shorter exposures. The trade off? More noise. In order to properly expose your shot, you'll have to bump up the ISO to compensate for the shorter exposures. How much noise you can tolerate is up to you! Let's see some examples, shall we?
The above image was shot at ISO 12,800, F/2.8, 10-seconds. You can see clear definition in the pillars. However, check out the noise levels. What happens if you push this further?
In this image, I bumped the ISO to a rather insane 25,600. This 4-second exposure, also shot at F/2.8 yielded much more definition in the pillars. At the cost of noise, though! I, personally, don't care for that amount of noise.
The following set of images show the differences in exposures done with a Nikkor 70-200mm telephoto lens. The longer shots were tracked to keep the stars from trailing.
10-seconds - F/2.8 - ISO 4000
20-seconds - F/2.8 - ISO 4000
30-seconds - F/2.8 - ISO 4000
60-seconds - F/2.8 - ISO 1600 (Because this shot was so much longer, was able to drop the ISO and thus end up with a cleaner image)
Can you see a difference? As the shutter increases, the motion gets more and more blurred. Sometimes you'll loose some pillars altogether. The image itself isn't bad, per se. It comes down to your own personal preference on if you like the more "dream-like" blurred motion, or the contrasty, razor-sharp pillars.
Also, if you noticed the foreground was the same across all the images, kudos! I used an untracked foreground for all of those shots - to keep it cleaner.
Now that we discussed shutter speed and its effects, let's discuss techniques. The most simple is single images. You set your camera up, take a shot, and that's it! Adjust settings as needed, and shoot until you run out of card space or get tired.
To get started, I always recommend these settings for a wide-angle lens - ISO 6400, 15-seconds, and whatever your lowest F-step is. F/2.8 and lower is preferred. This will almost always get you in the ball-park. Each camera and lens combo is different, so 99% of the time, you'll need to adjust accordingly. That is really all there is to it, though!
As the name implies, tracked images employ the use of a star tracker. These allow for longer exposures of the sky since the tracking mount moves with the earth's rotation. The downside to tracking is that you will almost always need to shoot a separate foreground shot. I say "almost" because at 15-30 seconds the motion may not be that noticeable. There's also tracking at 1/2 speed that helps balance the foreground and sky motion more. If your goal is shooting for web use, this is a viable option. If you are shooting for print though, you'll notice the star motion.
Tracking also requires proper alignment with the Polaris (northern hemisphere) or Sigma Octantis (southern hemisphere). While not terribly difficult, this extra step can be off-putting for some. Note that the alignment can be quite rough and still work fine with wide-angle lenses.
Settings with the use of a tracking mount vary widely based on personal preference, conditions, and gear. For example, on a very windy night, it may be impossible to shoot longer than 1-minute. I typically start with these settings - ISO 4000, 20-seconds, F/2.8. Once you confirm your tracking is good, adjust accordingly.
Stacking is the process of taking a series of images shot in sequence at a high ISO and combining them to mitigate noise. This process is shockingly effective and an wonderful option for those who don't want to invest in a tacking mount or for areas where fitting a tracker is impossible. This is different that focus stacking, where you take multiple exposures at different focus points to create a a sharp image from front to back. You can also employ both type of stacking if you want!
There are several programs out there designed for noise-related stacking. I use a free one called Sequator. If you are a Mac user, you'll want Starry Landscape Stacker. More involved programs such as Deep Sky Stacker can also handle it, but are more complicated as they are designed for the more complex imaging found with deep sky objects. Not so much for wide-field images.
Here is a peak into Sequator. On the left, you'll see the list of images you want to stack. The computer will assign one image as the base reference, or you can pick. There's also a place for dark frames and the like. I have never used them, though.
Below the first box, you'll see a menu for different options. The ones I have highlighted are the ones I *almost* always use. Note the "freeze ground" option towards the bottom. This program allows you to mask out the sky from the foreground, and then it will "freeze" that in place for you. While this maybe good enough for some applications, I typically use another lower ISO shot for the foreground. Or in the case of this shot, a focus-stacked version. I'll still tell it to freeze the foreground though, as that'll be easier to blend anyways. Starry Landscape Stacker operates in a similar way.
The following images illustrate the power of stacking vs. a single exposure. For this image, it worked out great because there was not a whole lot of definition in the Aurora on the horizon to begin with. Much like tracking, stacking will net a loss in pillar detail.
Twelve image stacked shot with a focus stacked foreground. Notice the amount of color detail that could be pulled, as well as the overall noise reduction. Stacking an such a powerful - and free - tool at your disposal! Highly recommend it!
Single image exposure blended with the focus-stacked foreground, for comparison.
So, What's "Right?"
As I stated in the beginning of the article, there is no right or wrong way to photograph the Aurora. Each night is different and thus may call for one method over the other. My advice is to get out there, enjoy the night sky, and play around for yourself! Regardless the path you pick, the adventure and experience is as mesmerizing as the images themselves.
Hope you found this article somewhat informative! As always, if you have any questions feel free to email me. Thank you for the continued support and happy shooting!
I'm not even opposed to extremely long single image exposures for star trails. This is a 5-minute exposure from last year's awesome Labor Day show.