I used mostly the Samyang 14mm f2.8 on a D800 to shoot these. Exposure times were in the 15 to 30 second range and ISO was 1600. As the aurorae became more active, the exposures were shortened to avoid blurring details from the bands.
There's quite a bit of spherical aberration in the centre of the image up until f4 or so. Bizarrely it goes away in the corner of the frame. Strange lens I must say.
Below is a panoramic view to the north sky, about two hundred degrees wide field of view. This particular image will be printed onto the pages of next Tähdet ja Avaruus magazine, biggest astronomy related paper in the Scandinavia.
Yes, the tree is prominently in many of the shots, but there wasn't anything more interesting to give scale to the lights since this was in middle of nowhere. Here's one shot with 50mm lens:
Nad last but not least, here is a spherical panorama from the observation spot, note the Astrotrac device in the zenith of the panorama; even at 14mm, the stars become trails in 30s exposures - more in the corner of the image due to the ultrawide rectilinear projection.
Northern lights and the Milky Way in Maaninka in Finland