Astro exposure (star-trail limit)
The Earth turns, so past a certain shutter speed pinpoint stars become short trails. Enter your lens to get the longest safe exposure by the classic rules.
The 500 rule is the classic limit; 300 is stricter for critical sharpness. Stars near the celestial pole (high declination) move slower, so they allow longer exposures.
About this tool
This tool gives the longest shutter speed you can use before the Earth's rotation turns pinpoint stars into short trails. Enter your focal length, film format and the target's declination, and it returns the limit by the classic 500, 300 and 200 rules.
How to use it
- Enter the focal length of your lens in millimetres.
- Choose your film format so the tool uses the right equivalent focal length.
- Set the declination of your target, higher near the celestial pole.
- Read the maximum exposure for the 500, 300 and 200 rules.
FAQ
Which rule should I use?
The 500 rule is the classic limit for a natural look; use 300 or 200 for critically sharp stars, especially with fast, wide lenses.
Why does declination matter?
Stars near the celestial pole trace smaller circles and move slower, so they allow a longer exposure before trailing.