Star Trail Calculator

In short The 500 rule says the longest exposure before stars visibly trail is 500 divided by the full-frame equivalent focal length, so a 24 mm lens gets about 21 seconds. It is easy to remember and it is out of date. It was written for film and for the first six-megapixel DSLRs, and it takes no account of how finely the sensor samples the image — which is the thing that decides whether a trail is visible at all. On a modern high-resolution body it is badly too generous: at 61 megapixels the same 24 mm lens really allows about 8.8 seconds, so the 500 rule permits a trail nearly two and a half times longer than a point. The NPF rule does account for it. It is (35 times the f-number plus 30 times the pixel pitch in micrometres) divided by the focal length, and because pixel pitch comes from the resolution, it knows what the 500 rule cannot. Enter your megapixels above and this tool shows both and tells you the gap. Two refinements. Declination matters: a star near the celestial pole traces a much smaller circle than one on the celestial equator, so you can expose considerably longer on it, and both rules assume the worst case. And all of these are judgements about an acceptable amount of trailing rather than physical limits — it is the same circle-of-confusion question as depth of field, and it changes the moment you decide to print bigger. The real answer for serious work is to stop guessing and stack: many short frames averaged together beat any single-exposure rule, and remove noise at the same time.