How to Avoid Star Trails in Astrophotography
Because the Earth is constantly rotating, stars appear to move across the night sky. If your camera's shutter remains open for too long, the stars will blur into lines (star trails) rather than remaining sharp pinpoints of light. To prevent this, photographers rely on mathematical formulas to determine the maximum exposure time.
The Classic 500 Rule
For years, the 500 Rule has been the golden standard. The formula is simple: divide 500 by your lens's full-frame equivalent focal length. For example, using a 20mm lens on a full-frame camera allows for a 25-second exposure. However, with the rise of modern, ultra-high-resolution sensors, the 500 Rule often results in slight trailing when zooming in at 100%.
The Advanced NPF Rule
Developed by Frédéric Michaud, the NPF Rule is a much more rigorous and accurate formula. It takes into account not only the focal length but also your camera's aperture and the physical pixel pitch of the sensor. A wider aperture and a larger physical pixel pitch allow for slightly longer exposures before trailing becomes noticeable on a pixel level.
Recommendation: Use the Classic 500 Rule if you are shooting exclusively for Instagram or viewing on mobile phones. If you plan to crop your image, print it in large formats, or if you are using a 40+ megapixel camera, strictly adhere to the NPF Rule limit.