I photographed the aurora borealis in Iceland during one of my winter photo tours there. The typical exposure I use is 13 seconds at f/2.8 and 640 ISO. If the maximum aperture on your lens is f/4, instead of doubling the exposure time to maintain the correct exposure, it’s better to double the ISO. A longer exposure time makes the stars into oblong shapes as they approach the beginning of a star trail. This was taken with a 16mm focal length. Since it's impossible to focus on a night sky, I placed a small flashlight on the ground about 35 feet away and focused on that. For a 16mm lens, this distance is beyond the point where the lens goes to infinity, so focusing correctly on the light produced sharp stars. It's essential, though, to switch to manual focus before you shoot so the camera doesn't attempt to focus on the aurora.
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