To achieve this however, you need some money. That is to say, fast lenses are usually expensive. In this case, I used Canon’s 24mm f1.4L, it goes for about $1300. If a 24mm f2.8 lens was used–same focal length but two stops slower–the exposure would have been 20 seconds. That would have rendered a different looking image indeed, lacking the detail in shape. This is especially true when the aurora is moving quickly. The focusing of this lens however, is very finicky and I’ve had many frustrations with slightly blurry images. In contrast, my 16-35mm f2.8L, focused in the exact same manner, in the same conditions, works beautifully. I’m often shooting two cameras, one with the 24 f1.4 and one with the 16-35mm f2.8.
There is much to say regarding Aurora borealis photography. I’ve written a brief and basic article, which can be read in full here: “How to photograph the aurora with a digital camera“.