Tuesday, 3 February 2015
Often I am asked how to take photos of the Northern Lights. How does one need to set up a camera to be successful? There are, of course, other considerations as well, but here I want to concentrate on the main parameters that determine how sensitive the camera is to light. If your camera can operate using the values given in the chart below, you will be able to photograph the aurora.
Focal length is the least important factor discussed here. The Northern Lights often span the whole sky and thus you want to get as much sky as possible into the image. Therefore, zoom out as much as possible or choose a lens with a wide field-of-view. Focal length is a measure of field-of-view, and the smaller your focal length, the more sky you can see. Typical wide-angle focal lengths are 14mm to 35mm, but also 50mm might work for some scenes (for a full-frame camera). For a crop-sensor (APS-C) camera or a micro-four-thirds camera you need to multiply these numbers by 1.5 or 2, respectively, and the resulting number should stay below 50 mm.
For nighttime photography of the sky, the camera needs to let in as much light as possible. Therefore start by setting the aperture as widely open as possible. Apertures are given as ratios like f/2.8. The aperture is largest, when the so-called f-number is smallest. For instance, an aperture of f/2.0 will let more light onto the camera sensor than an aperture of f/2.8. In other words, start by choosing the smallest f-number.
Exposure Time and ISO
Next, you have to balance between exposure time and ISO value, which are directly coupled together for a fixed aperture. For the same scene, doubling ISO requires halving exposure time, or halving ISO requires doubling exposure time. For example, an exposure time of 4 seconds at ISO 400 is the same as an exposure time of 2 seconds at ISO 800.
In other words, ISO determines how light-sensitive your camera is. The problem, however, is that the higher you set the ISO sensitivity, the noisier and grainier the image will become. It depends on your camera model – and your taste/judgement – how much image noise you want to accept. Take your camera out at night and try it with different settings for a few scenes to find out what the highest acceptable ISO value is for your camera.
Also for exposure time there are upper limits. First, if the Northern Lights are really active and fast moving, one needs to keep the exposure time as low as possible, otherwise all the motion will make the northern lights in the photo look very blurry without any structure.
Secondly, at some point the rotation of the Earth will cause the stars in the image to appear as lines rather than dots. A commonly used rule states that one should keep the exposure time below a value of 500 divided by focal length (in seconds). For example a lens with a focal length of 50 mm will allow for a maximum exposure of 500/50 = 10 seconds. Note that, if you use a crop sensor (APS-C), you have to multiply the focal length by 1.5 or 1.6 depending on your camera.
Balancing Exposure, ISO and Aperture
The diagram above (click it for a larger version) will help with this balancing act. The dashed horizontal lines at the top give the maximum exposure time that will keep the stars as dots for a number of popular focal lengths. Check where your lens reaches its limit here at its widest field of view (shortest focal length).
Next, choose the diagonal line with the f-number of your widest possible aperture. Then you can move along this line to see which combination of ISO values and exposure times to start with when photographing the Northern Lights.
For example, if your lens is a 28mm f/2.8 on a full-frame camera, then you can go to a maximum exposure time of 15 seconds, and an exposure time of 4 seconds at ISO 1600 or 2 seconds at ISO 3200 would work fine.
The photo above was taken at only 0.5 seconds exposure time at ISO 800 using a 24mm f/1.4 lens. This shows that the graph is not a definite rule. The graph provides a starting point, meaning if you set your camera using these values, you will succeed taking photos of the Northern Lights. But if the lights are bright, you might get away with shorter exposure times or lower ISO values, thus you can freeze faster motion or get less sensor noise.
Finally, this chart will tell you also if your equipment is enough for the job. Just check where your camera specifications would be on this chart. Furthermore, modern camera sensors get more sensitive all the time, and some cameras nowadays get decent results with ever higher ISO values. Thus I encourage you to give it a try and see how far you can push your camera.
Photo and graph: Thomas Ulich.