While not inexpensive, at $1,599, it seemed cheap compared to lenses with similar, but not quite as good specs that are often more than $2,000. I purchased the Sigma with my own money, because nighttime photography is a big part of my photo output. It's available in L Mount (Leica, Sigma, and Panasonic cameras) and the Sony E Mount.
Here's some of the tech info on this lens. First, and foremost, as far as I know, this is the first and only full frame lens that can combine an ultra-wide 14mm optical system with a maximum aperture of f/1.4.
The sky is difficult, because your image contains thousands of small points of light, often spread all over your image. In the real world, it means that with most lenses, even if the center of the image looks good, the corners look awful, with stars stretched into sausages. They are no longer points of light.
The Sigma also features high-speed autofocus with HLA (High-response Linear Actuator) linear motor, although in my own work under the stars, I prefer manual focus, and I'll talk about that below. For landscape photography, even in sunset or sunrise light, the autofocus is very useful.
The Sigma is a complicated piece of gear, and it has to be to get these kind of results. It contains 19 elements in 15 groups, including one SLD glass element, three FLD glass elements, and four aspherical lens elements.
This Sigma Art Lens comes with a detachable tripod socket with an Arca-Swiss type connector. The lens is very heavy, and it sticks out so far from my Sony a7 IV, I was worried about it tipping over. Using the tripod socket balances things nicely and changes the center of gravity.
The lens includes an all important autofocus lock button and an aperture ring. The body is dust- and splash-resistant, and the front-most surface of the lens is coated with a water- and oil-repellent coating so you can shoot outdoors in harsh environments. According to Sigma, "The HLA (High-response Linear Actuator) enables high-speed, quiet, and highly accurate autofocusing. In order to maintain performance while supporting a lens with a large aperture, the lens has a robust internal structure and uses lightweight materials such as polycarbonate TSC (Thermally Stable Composite), which has a thermal shrinkage rate equivalent to that of aluminum, and magnesium in appropriate locations to reduce weight while ensuring robustness."
As I said, it's a heavy lens, but it's built like a tank, and I expect years of good service.
In the Field
After weeks of brutal monsoon weather in Southern Arizona, I finally got my camera with the Sigma out for a night of Milky Way photography. We have Bortle 4 skies here away from Tucson and Phoenix, and to see the Milky Way, all you have to do is look up.
I hooked my camera body up to my Peak Design travel tripod, and standing under the night sky, I had an awful lot of anxiety about the weight of that heavy lens. My equipment and I both survived, but next time, I'll use the tripod socket. I tried it in the backyard the next day on an even lighter tripod, and the balance was perfect.
Back to my night under the dark sky: I pointed the camera to the south-southwest, where the Milky Way was almost vertical. My usual practice is to shoot 20-40 images. 13-second exposures work well with my camera and lens combo. I do get some sky glow from Tucson low in the sky to the Southwest, but Tucson, home of many observatories, takes steps to keep lighting limited.
I loved focusing the Sigma lens using the LCD screen on my camera. As soon as I touched the focus ring, the stars magnified and focusing manually was easy. Then, I flipped the Focus Lock switch, and I was set for the night. Autofocus sometimes worked, but it's difficult with stars. Usually, when I don't focus manually, I try to focus on a distant object before it gets too dark, but it's not as good as doing it manually when focus is so obvious in the magnified image.
I took three sets of images, some with only the sky and the Milky Way to try to minimize the light pollution glow near the horizon, and I took some pictures that put the Milky Way framed with some cactus.
I processed the images in Starry Landscape Stacker for Mac, so when I was done, I had one stacked image with the foreground and background in focus. On the Windows side, there's a similar app called Sequator.
What's the Competition?
For my camera, the Sony a7 IV, I've had the venerable Rokinon 20mm f/1.8. It's a very nicely priced lens, available for just about every camera mount. At that low price, you're going to get a field that is a bit distorted, but Photoshop and DXO provide software fixes for that. It's not a heavy lens, and the Rokinon (sometimes sold under the Samyang label) will do the trick with some flattening and cropping. They can be purchased for a range of about $350 to $499. Now, Rokinon lenses appropriate for night sky use are available in several fast focal lengths and aperture sizes. There's a variety of prices, and all are a good lens at low cost.
I also have what is often touted as a great Milky Way lens from Sony, their 20mm f/1.8 G lens. It is a terrific lens, light in weight, especially compared to the Sigma, and it presents a nice flat field with minimal distortion. It has that all-important focus lock feature on a button, but it has to be programmed via the camera. It's about half the price of the Sigma at $799, and a fine lens. (Sony now offers an FE 14mm f/1.8 lens that is within a hundred dollars of the Sigma. But it's f/1.8, not f/1.4.)
Still, the Sigma is an even better night sky lens. It has less distortion, faster optics, and a wider field of view than the other contenders I've used. The only drawback is its weight (4.2 lbs!), but as I noted above, it can be dealt with. Shooting at f/1.4 means less time in the elements because you can shoot fewer frames by gathering more light. I always shoot wide open with the Sigma, and other lenses I've used want you to stop down some to keep the stars from elongating at the edges.
You do get just a little distortion out at the very corners, but at normal resolution, it's invisible and better than other night sky lenses I've used. If you image at say, f/2 or f/2.4, the edges will be tack-sharp, but I'd rather have the faster performance.
For Milky Way, night sky, even wide field landscapes, the Sigma is really superb. If you do night sky astrophotography or landscape work, it is worth a serious look for Sony or L Mount camera owners, and my hope is Sigma will make versions for other popular cameras.