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500 Rule Exposure
10.0 s
500 Rule Exposure vs Focal Length
Formula
## Astrophotography Exposure Rules ### 500 Rule (simple) **t = 500 / f** seconds (f = focal length in mm) Stars will begin to trail after this time on an untracked mount with a full-frame sensor. ### NPF Rule (more accurate) Accounts for pixel pitch and is better for modern high-resolution sensors. A shorter exposure may be needed for sharp pinpoint stars.
Esempio Risolto
50 mm lens with 4.3 µm pixel pitch.
- 01500 Rule: 500 / 50 = 10 seconds
- 02Stars trail after about 10 seconds at 50 mm.
- 03For crop sensors, divide by the crop factor (e.g., 1.5).
Domande Frequenti
What about crop sensor cameras?
Use the effective focal length (actual × crop factor). For a 50 mm lens on APS-C (1.5× crop): 500 / 75 = 6.7 seconds.
How do I take longer exposures?
Use a star tracker (equatorial mount) that rotates to compensate for Earth's rotation. This allows minutes-long exposures.
Why do stars trail?
Earth rotates 15 arcsec/second. At long focal lengths this motion is magnified, causing stars to streak across the sensor.