Star Trail 500 Rule Calculator
When photographing the night sky with a static camera, Earth's rotation causes stars to appear to move, creating trails in longer exposures. The classic 500 rule gives a quick maximum exposure time: 500 divided by the full-frame equivalent focal length. This calculator applies the rule automatically based on your actual focal length and crop factor, and also provides the stricter 400 rule (preferred for modern high-resolution sensors) and the NPF rule for even greater accuracy when you know your sensor's pixel pitch.
500 rule formula
Max exposure (s) = 500 / (focal length x crop factor)
400 rule: Max exposure (s) = 400 / (focal length x crop factor)
Example: 24 mm lens on a 1.5x APS-C crop sensor. Equivalent focal = 24 x 1.5 = 36 mm. 500 rule: 500 / 36 = 13.9 s. 400 rule: 400 / 36 = 11.1 s. For high-resolution sensors over 24 megapixels, the 400 rule or NPF rule is recommended to avoid visible trailing.
Which rule should you use?
- 500 rule: original guideline, works well for sensors up to about 18 MP. Quick and easy.
- 400 rule: preferred for 24-45 MP sensors where the 500 rule can show subtle trailing on large prints.
- NPF rule: most accurate, accounts for pixel size. Useful for cameras over 45 MP or when maximum sharpness is needed for astrophotography stacking.
- Stars near the equator trail faster than polar stars. These rules give conservative estimates for equatorial stars.
Star trail 500 rule: frequently asked questions
What is the 500 rule for astrophotography?
The 500 rule states that the maximum exposure time before stars start to trail is approximately 500 divided by the full-frame equivalent focal length. At 25 mm equivalent, the maximum is 500/25 = 20 seconds. This is a quick rule of thumb for full-frame cameras.
Why does focal length affect star trailing?
Stars appear to move across the sky due to Earth's rotation at about 15 arc seconds per second. A longer focal length magnifies this motion more, causing stars to trail more quickly across the sensor. Shorter lenses give more time before trailing becomes visible.
What is the NPF rule?
The NPF rule is a more accurate formula: Max time = (35 x aperture + 30 x pixel pitch) / focal length. Pixel pitch is sensor width / pixel count on one axis. It accounts for pixel size, which the simple 500 rule ignores.
How does crop factor affect the 500 rule?
On a crop sensor, stars move faster across the frame in angular terms (same sky motion but smaller field of view, so more pixels per degree). Use the 500 rule with the full-frame equivalent focal length (actual focal x crop factor) for a rough calculation.
What is a typical maximum exposure for astrophotography?
At 24 mm equivalent on a full-frame camera: 500/24 = about 21 seconds. At 50 mm equivalent: 500/50 = 10 seconds. At 14 mm on APS-C 1.5x crop (21 mm equivalent): 500/21 = about 24 seconds.
Official sources
- ISO 12232:2019: Photography, Digital still cameras, Determination of exposure index. ISO.org.
- US Naval Observatory (USNO): Earth rotation and celestial reference. USNO.navy.mil.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.