About shooting auroras
I always considered the lens to be the most important material in my photo bag.
In 2018, while shooting auroras with a guide from Chasing Light, she discussed cameras and lenses and showed my the lens she was using. A Sigma 20mm f/1.4. I bought this lens before I made my second journey to Aurora latitudes. Yes, it's a very heavy lens, that you will feel when carrying on your back for hours. No you also cannot attach a filter to it, unless I guess using one of those specific and expensive systems, which I'm not even sure exists for that particular lens.
But, what an incredible piece of glas and technology!
I also own a 16-35 f/2.8 from Sony. I took pictures with both of them in my last Norway trip. Cleary, the Sigma is a better option.
1) It's lightning fast! The Auroras constantly move, so if the expo time is too long, you end up with a big green zone on your image, which doesn't do justice to the aurora's dancing. The f /1.4 aperture of the Sigma lens is a killer. It allows you to capture light like no other lens I've tried. On the pictures below, you can see that the moon is lightning the mountains, it feels like it was almost daylight, the Auroras is sharp, the the stars are sharp, and that's because the expo time was only 5 seconds. You take that photo with a f/2.8 and you double the time, or you have the cramp up the ISO, and end up with a noisy image. The ISO I used with the Sigma is 640. On the Sony A7iii, you barely see any noise at all. When you shoot at 3200, it's another story. In two words, I don't want muddy auroras, therefore I shoot with a very fast lens, and the Sigma is affordable and my perfect solution for getting sharp and bright pictures of northern lights.
2) It's incredibly sharp, and really belongs to the high quality lenses produced by a very solid and well recognized company.
3) It's not expensive! (relatively to other lenses, I mean). The price is around 700 Swiss Francs, which in comparison with my 16-35mm is nothing, trust me! Ok, it's not the same lens, but for shooting auroras, it's the best option in my opinion.