Astrophotography 500 Rule Calculator
Estimate the longest shutter speed for stars before visible trailing appears.
About this tool
An astrophotography shutter calculator using the 500, 400 and 300 rules. Enter focal length and sensor crop factor to estimate a star-trail-safe exposure time.
Frequently asked questions
What is the 500 rule?
The 500 rule estimates max shutter speed as 500 divided by focal length and crop factor, giving a rough limit before stars visibly trail.
Should I use 500, 400 or 300?
500 is generous. 400 is safer for modern sensors, and 300 is stricter for high-resolution cameras or larger display sizes.
Does crop factor matter?
Yes. Multiply focal length by crop factor because smaller sensors narrow the field of view and reveal trailing sooner.