One common mistake that’s often made by pros, beginners and everything inbetween is when starting to shoot the nights sky; leaving the shutter open way too long causing star trails (more on that later).
Sometimes we like or want this effect but generally we don’t.
So how do you work around that issue of trails and overly bright sky, well we can start with a little bit of maths…….
500 divided by the focal length of your lens “Equal to” The longest exposure before stars trail in the final image.
While this rule isn’t set in stone it should give you the basis of how to start shooting the night sky with minimal, or at least next to zero trails. Depending on the camera you may want to try starting with 400 or 600 to divide by but start at 500 and see where that takes you, if you have a crop sensor then try the Rule of 400 first as that has a multiplier of 1.3 to 1.5 and will have a different result anyway.
So for instance, if you have a prime lens such as a 15mm (a popular lens for night sky shooters, one I like to use all the time) on a full frame camera. Try 500 divided by 15 = 33.3 seconds, you can be a little flexible here and attempt 30 to 34 seconds as a fine tune and depending on your kit. If you see trails then reduce your exposure time, if the scene gets a little dark then bump the ISO slightly to compensate.
Other lenses to try are a 24mm, 40-50mm as well.