Photographer Ben Canales created this great video tutorial teaching the basics of shooting the night sky. He goes over how to shoot quick test shots to set up your composition before discussing more in-depth tips and tricks for capturing the final shot, including the “Rule of 600″:
[…] the quickest way to determine the longest exposure that is possible for any given focal length lens, without the stars streaking, is to divide that focal length into 600. (This is the formula for 35mm. Larger formats are laxer, smaller formats more unforgiving). [#]
For example, with a 50mm lens on a full frame camera, you can only expose for 12 seconds (600/50=12) before the stars turn into star trails. It’s a good rule of thumb to keep in mind!