Calculate star trail

Compare NPF and 500-rule exposure limits for keeping stars point-like and estimate trail rotation.

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ToolStar Trail Calculator
Input
Output
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Preview

How it works

The NPF limit uses (35·aperture + 30·pixel pitch) ÷ focal length, while the 500 rule divides 500 by focal length and crop factor. Trail length then follows exposure duration and apparent sky rotation.

  • Both limits are heuristics, not guarantees.
  • Sensor format and pixel pitch affect the tolerance.

Worked example

24mm f/2.8 on a 4.3 micron pixel pitch full frame body
A common wide astro setup
Input
											Focal length mm: 24
Aperture: 2.8
Pixel pitch um: 4.3
Output
												Npf seconds: 9.46
Five hundred rule seconds: 20.83
Recommended seconds: 9.46
Crop factor: 1

When to use this

Night photographers choose limits, timelapse plans compare rules, and astronomy lessons relate sky rotation to trails.

Edge cases

  • The 500 rule can allow visible elongation on high-resolution sensors.
  • Wide lenses reduce angular motion per pixel but do not stop Earth’s rotation.
  • Tracking mounts, stacking, and light pollution change the practical plan.