About this tool
500-Rule / NPF Star Trail Calculator is a free, in-browser tool for nightscape and Milky Way photography. Choose your sensor, focal length and (for the NPF rule) aperture and horizontal pixel count, and it returns the longest shutter time you can use before stars start to streak — by both the quick 500 rule and the sharper, pixel-aware NPF rule. All math runs locally with nothing uploaded.
The 500 rule is maxSeconds = 500 / (focal length × crop factor), a fast field estimate. The NPF rule accounts for aperture and pixel density: pixel pitch = sensor width in mm × 1000 ÷ horizontal pixels (in microns), and the time is (35 × N + 30 × pixel pitch) / (focal length × crop factor). The NPF rule is stricter, so it yields shorter exposures that keep stars point-like on high-resolution sensors.
Use it to set your exposure at the star field: the 500 rule for a rough starting point, and the NPF rule when you are pixel-peeping and want genuinely sharp stars in the corners of a large print.