Here’s the F-Stop to Use

This is an easy one: use the lowest numbered f-stop your lens will allow. So, if your lens will let you go as low as f/2.8, or even f/1.8, then there ya go. Now, the lowest f-stop your lens will go might only be f/3.5 or f/5.6, but whatever that number is, that’s the one to use. The stars themselves don’t produce a whole lotta light, so you’ll need all the help you can get (plus, using a fast f-stop like this will help keep you from having to bump your ISO up too much to where you start to introduce a bunch of noise). So, the lower the number, the better, when it comes to f-stops.