Corner sharpness isn’t a concern either, because if you shoot the night sky there’s nothing of interest in the corners to start with. Bottomline: Shooting the night sky at maximum aperture isn’t a big deal. The wider the aperture, the better – as we are going to see shortly.
Hang on a second, why shoot at maximum aperture if we can just pick a slower shutter speed? That should fix it, right? Unfortunately it does not, because Earth is rotating around its axis while everything else is standing still (that’s not quite correct, but it will suffice for the purpose of this article). Consequently, objects in the night sky seem to be moving from our point of view and the speed at which they are moving is noticeable, even more so as we crank exposure time up. For the Milky Way we need to follow the 500 Rule:
Here t is the slowest possible shutter speed and f is the focal length. For my previous example (14mm) we get
I usually pick a 30s shutter speed and that has worked out nicely so far. If you try to expose for longer you’ll get trails, which in some cases may be desirable but should be avoided for shooting the Milky Way.
We’re slowly running out of options, in fact, we only have but a single option left: Changing the ISO. If you’re a seasoned landscape photographer you know that there’s not much wiggle room left here either, as noise will ultimately be a limiting factor. As a rule of thumb I don’t usually go any higher than ISO 3200 on my D850.
As you can see, there aren’t exactly many degrees of freedom, your best bet is as fast a lens as possible. For example, I happen to own an AF-S Nikkor 24mm/1.4G ED wide angle lens as well, so in a pinch I can gain 2 full stops, which can make a huge difference in image quality (ISO 3200 instead of ISO 12800 is massive).
Phew, that was a lot of theory, but we finally got camera settings pinned down. Now we need to turn our attention to location, time and weather – just like we need to in “conventional” landscape photography.
This can be a tough one, depending on where you live. If you already had a location in mind, there’s a good chance the shoot is not going to happen, because there’s too much artificial light around. This is aptly called light pollution and will appear as a “fake sunrise” (orange glow on the horizon) in your pictures, sometimes completely obliterating the center of the galaxy.
You can, of course, give it a try and see for yourself how bad it is, but if you have to travel some distance a wiser course of action is to check on a light pollution map and make sure you’ll get something for the time and money you’re going to sink into this attempt. You can either search for locations on the web or install an app on your mobile device, see for example Light Pollution Map (Android) or Light Pollution Map (iOS).