The 500 Rule in astrophotography Definition & Meaning The 500 Rule in astrophotography is a guideline to determine the maximum exposure time (in seconds) to avoid star trails caused by Earth’s rotation. It is calculated as: \[ \text{Exposure Time (seconds)} = \frac{500}{\text{Focal Length (mm)} \times \text{Crop Factor}} \] 500 is a constant. Focal length is… Continue reading What is the 500 Rule in Astrophotography? Meaning & Examples