If you’ve tried astrophotography you’re probably very familiar with how difficult it can sometimes be to focus on the stars. The general process is to find a bright star, point the center of your lens at it, find it in your live view, and turn the focus ring until the star is small and sharp. When doing this, use the largest aperture on your lens and the highest ISO to get the brightest video feed in your live view.
With practice this becomes pretty easy (and is how I do it most of the time), but on some lenses it is very difficult to tell when you are in perfect focus on the stars or just a tad off. Being a little off may or may not be a big deal depending on the lens. It could be fine, or it could result in excessive chromatic aberrations (or other aberrations) around the stars, and it’s hard to tell that until you’re at home looking at the full resolution raw file on your computer.
On some cameras there are features to make this easier, including point focus, focus masking, and boosting the video feed to see the stars more easily. For example, on at least some Nikon cameras if you put the camera into video mode and choose the longest shutter speed and highest ISO you will have a much better view of the stars than when using live view in still photo mode.
There are also tools available that are specifically designed to get perfect focus on the stars. They are called Bahtinov masks. You may have heard of or seen these, they are most commonly round pieces of plastic or metal with slats inside of it. The design diffracts the star light entering your lens, and in your viewfinder or live view you will see a star like diffraction pattern like the one above. However these types of Bahtinov masks are designed for telescopes and long telephoto lenses, they don’t work on shorter focal lengths.
For a shorter focal length you need a Bahtinov mask designed for such a purpose. I mention the SharpStar2 in my book, Night Sky Photography: From First Principles to Professional Results, but since I wrote the book a new product has come on the market called Focus On Stars. This does the same thing as the SharpStar2 but it is much better. With the SharpStar2 it is very difficult or impossible to see the diffraction spikes in live view even on the brightest stars on a super wide angle lens, e.g. 14mm or around there, which means you take your best guess at focusing and then take a test shot with the filter in place to confirm focus. But the Focus On Stars mask makes it much easier to see the diffraction spikes. You can see examples of the diffraction spikes that show up in test shots with each mask in the image below. Even with a test shot on a wide angle lens, the SharpStar2 makes it difficult to see the diffraction spikes, but they are clearly visible with Focus On Stars.
It still takes some practice, and you might need to move the camera and mask around to get the spikes to show up their best in live view, but it’s actually possible to see the diffraction spikes with the Focus On Stars mask using live view. In either case, you still need to take a test shot with the filter in place to confirm accuracy, but it’s much easier with the Focus On Stars mask.
You can hold the filter in place with a filter holder, or you can just hand hold the filter in front of the lens if you have a reasonably steady hand, it doesn’t need to be perfectly still.
So if you’re having trouble getting focus on the stars, you might consider trying a Bahtinov mask. That said, I only ever use one for long focal lengths and/or very fast (very wide aperture) lenses where getting focus is tricky. At 14-24mm or so (full frame), I’ve been doing it so long that I just do it by eye and take test shots to confirm.