Capturing the Stars: The Rule of 500
In astrophotography, the goal is often to capture the stars as sharp, distinct points of light. However, because the Earth is constantly rotating, the stars appear to move across the sky. If your shutter is open too long, the stars will 'trail,' turning into small lines instead of dots. The Rule of 500 is a simple mathematical formula used to determine the maximum shutter speed you can use before star trailing becomes visible.
The Logic of the Rule
The wider your lens, the less apparent the star movement is, allowing for longer exposures. A telephoto lens 'magnifies' the star movement, requiring a much faster shutter speed.
The Formula
Max Shutter Speed = 500 / (Focal Length * Crop Factor)
The Rule of 300 (The NPF Rule)
Modern high-resolution sensors are much more sensitive to star trailing than the film and low-res sensors the Rule of 500 was designed for. Many professional astrophotographers now use the Rule of 300 or the even more complex NPF Rule (which accounts for aperture and pixel pitch) for ultra-sharp results.