One of the goals I set for myself this year was to capture a decent aurora photograph. Many people reading this will know far more about aurora photography than I do, but I thought I’d jot down the basics here for those who are interested. The general principle is that you need to get as much light hitting your camera’s sensor for as long as possible (because it will be dark!), while keeping your camera very still. A camera with a manual mode such that aperture, shutter speed and ISO can be adjusted is required.
Aperture (f/stop)
This is a measure of the size of the hole through which light passes to the camera’s sensor (where the image is recorded). A smaller number equates to a bigger‘hole and thus more light hitting the sensor. I use a lens with a maximum aperture of f/2.8 and then manually set my aperture to f/2.8. Confusion often surrounds the f/stop and this sequence of seemingly random numbers (f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 etc). The f/stop number is actually the ratio of the lens opening to the focal length of the lens and is really an inverse amount, which accounts for its inverse relationship with the size of the hole. You don’t need to understand this, just set the number as small as your camera and lens will allow! This will give you the biggest hole and allow the most light to hit the sensor.
Shutter speed
Make this long so that there is sufficient time for the light to get through the aperture to hit the sensor. I typically use 15 or 25 seconds. Be aware that auroras move and you will essentially capture an ‘average’ image of where the aurora was over the time that the shutter was open (and the aurora’s light was hitting the camera’s sensor).
ISO
This is a measure of the camera sensor’s sensitivity to light. A bigger ISO number makes the sensor more sensitive to light. Hence, for a given aperture and shutter speed a higher ISO will result in a brighter image. However, the tradeoff is that the image will start to appear grainy at higher ISOs (where exactly this occurs depends on the particular camera/sensor). I really don’t like this grainy quality and use quite low ISOs (under 400). Generally a wide angle lens that includes as much of the night sky as possible is desirable. I use a Tokina 11-16mm f/2.8. There is no reason that you couldn’t use a longer (eg telephoto) lens if you wanted to. More important is the lens’ maximum aperture. Remember a small f/stop number means a bigger hole and allows more of that night sky to hit the sensor. A higher maximum f/stop number (smaller hole) means that you will need to compensate by increasing the time that the shutter is open, or by increasing the ISO. In fact, to get the same amount of light hitting the sensor at f/4 as f/2.8, it is necessary to double the time that the shutter is open, or double the ISO.A tripod is very handy. It keeps the image — particularly any objects in the foreground — as sharp as possible.
Many people use a remote to open the shutter (ie to take the photo) so that they don’t bump the camera and blur the image when they press the shutter release button. I get around this by setting my camera to take the picture 10 seconds after I press the shutter release button (as you would for a self portrait). This way I’m not bumping the camera when the shutter opens. Another consideration is focus. Your camera might hunt for a focal point and refuse to take the photo because it can’t find anything to focus on. Best to set it to manual focus and, as auroras are a long way away, focus on infinity (ie as far away as possible). You might want to play around with this if you desire objects in the foreground to be in focus.
None of this is hard and fast. These are general principles only. Much of the fun is in playing around with settings and, let’s face it, hitting on a bit of luck as well!
James Chappell
Photos in front of Rumdoodle hut
Nikon D3200
Tokina 11-16mm f/2.8. Focal Length 11mm
Aperture: f/2.8
Shutter speed: 25 seconds
ISO: 200