Although I have taken thousands of pictures of the sky through telescopes, I have only rarely taken photos of the sky with a regular camera and lens.
Stars, being points, are a really good test for a lens. If the stars show up as points even far into the corners, it is a very very good lens. It’s actually pretty rare to find such lenses; most lenses will show some amount of coma (a type of aberration that smears the light) as you move away from the center. (You can ‘stop down’ the aperture, which means make it smaller, to reduce aberrations by reducing the area of the lens used to just the central portion. But this has its own issues: you are capturing less light when means you need longer exposures. And if you do not have a tracking mount (I do not), then the stars form streaks as the earth rotates.
There isn’t a hint of coma or any other aberrations all the way to the corners on this (and the other test) shots.
Technical info: Taken with my new medium format camera, the Phase One XF and a 28mm lens. This is probably the best wide-angle lens I have ever encountered, which is why I wanted to see how well it performs on stars.
Even though this is a wide angle lens, and even though this is just a 10-second exposure at f/4.5, I could see some open star clusters and at least one galaxy—I did not look them up, at least not yet, to see what they were. But, again, a good indication that this is a great lens for night sky photos.
Note: normally, f/4.5 would be considered a bit slow for the task, but since the lens is ultra-sharp, it concentrates the light quite well, which overcomes the slightly slow nature of the lens design. Normally, you’d try to get something that was f/2.8 wide open aperture.
Speaking of stars forming streaks, I deliberately made a 20-minute exposure to get them. I included Polaris, which barely moves since it’s very near the north celestial pole, you can see it at upper left.
There was a driveway light to the right of this frame, you can see a bit of it illuminating some tree branches at bottom right. The red reflections come from that light.