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
Worked out from the sensor width and megapixels; type your camera's own figure if you know it.
Maximum exposure for sharp stars
12s
NPF Rule (f/2.8, 6 µm pixel pitch)
500 Rule
21s
Traditional formula
NPF Rule
12s
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 for your inputs: Full Frame (35mm), f/2.8, 6 µm pixel pitch
| Focal Length | 500 Rule | NPF Rule |
|---|---|---|
| 14mm | 36s | 20s |
| 20mm | 25s | 14s |
| 24mm | 21s | 12s |
| 35mm | 14s | 8s |
| 50mm | 10s | 6s |
| 85mm | 6s | 3s |
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)
Frédéric Michaud's formula: exposure = (35 × aperture + 30 × pixel pitch in µm) ÷ focal length (the lens's actual focal length). It accounts for sensor resolution and aperture, and gives shorter, more conservative times that hold up to 100% crops. Choose your aperture and sensor above (or type a pixel pitch) and the figure follows. Pixel pitch is worked out from the sensor width and its megapixel count.
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
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