Find the longest shutter speed before stars start to trail (500 rule).
The 500 rule divides 500 by the effective focal length to give the seconds before Earth's rotation visibly trails stars. Staying under the limit keeps stars as points rather than streaks when you do not want intentional star trails.
Max Exposure for Star Trails
Max shutter = 500 / (focal x crop factor)
Max shutter = 500 / (focal x crop factor) The 500 rule divides 500 by the effective focal length to give the seconds before Earth's rotation visibly trails stars.
Staying under the limit keeps stars as points rather than streaks when you do not want intentional star trails.
This calculator takes 2 inputs: Focal length, Crop factor. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.