Star Trails Calculator
Calculate maximum exposure for sharp stars, or find the right exposure for intentional star trails. Uses both the classic 500 Rule and the more accurate NPF Rule.
Calculate star exposure
Choose whether you want to avoid or capture star trails
Maximum exposure for sharp stars
17s
NPF Rule (more accurate for modern sensors)
500 Rule
21s
Traditional formula
NPF Rule
17s
More conservative
Tip: These are maximum values. For pixel-peeping sharpness, especially on high-resolution sensors, use the NPF rule or even shorter exposures. The 500 rule was designed for film and lower-resolution digital.
Quick reference: Maximum exposure for sharp stars
Exposure times in seconds before star trailing becomes visible
| Focal Length | 500 Rule | NPF Rule | ||
|---|---|---|---|---|
| Focal length | FF | APS-C | FF | APS-C |
| 14mm | 36s | 24s | 29s | 12s |
| 20mm | 25s | 17s | 20s | 8s |
| 24mm | 21s | 14s | 17s | 7s |
| 35mm | 14s | 10s | 11s | 5s |
| 50mm | 10s | 7s | 8s | 3s |
| 85mm | 6s | 4s | 5s | 2s |
Understanding the rules
500 Rule (Traditional)
Maximum exposure = 500 ÷ (focal length × crop factor)
Developed for film and early digital. Works well for web-sized images but may show trailing when pixel-peeping high-resolution sensors.
NPF Rule (Modern)
A more complex formula that accounts for sensor resolution and aperture. Gives shorter, more conservative times that hold up to 100% crops.
Factors that affect trailing
- Declination: Stars near the celestial poles move slower than those near the equator
- Direction: Pointing north/south shows less trailing than east/west
- Viewing size: Larger prints/screens reveal trailing more easily
Plan your night shoots with Silverlog
Coming soon