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What the 500 rule is
Night photographers face a simple enemy: the Earth turns. During a long exposure, stars move across the frame and smear into short streaks instead of staying points. The 500 rule is the classic shortcut for avoiding that. Divide 500 by your focal length in millimetres, and the answer is the longest exposure in seconds that keeps stars looking like points. A 24 mm lens gives about twenty-one seconds, a 50 mm lens ten seconds, and a 200 mm lens two and a half seconds.
The rule works because it links two things: longer lenses magnify the sky, so they magnify the stars’ motion too, and longer exposures give that motion more time to show. A wide lens forgives a long exposure because each star moves only a tiny distance across a wide field of view. A telephoto lens forgives almost nothing. The calculator also adjusts for crop factor, because a crop sensor uses only the centre of the lens image and therefore magnifies the motion just as a longer lens would. Enter the lens focal length and your sensor type, and it does the adjustment for you.
Why the rule is only a starting point
The 500 rule was born in the film era, when prints were small and grain hid small amounts of trailing. Modern sensors changed that bargain. A high-resolution sensor records far more detail, so a trail that was invisible on film now shows clearly when you zoom in or print large. That is why the calculator also shows the stricter 300 rule: divide 300 by the crop-adjusted focal length instead. For a 24 mm full-frame lens the 300 rule gives twelve and a half seconds instead of twenty-one. If you plan to print large or crop hard, the stricter figure is the honest one.
Your own standards matter too. A photo for social media, viewed small on a phone, forgives more trailing than a large print on a wall. A photo you will crop to half its size forgives less, because cropping magnifies the trails just like a longer lens. There is no single correct number, only a trade-off between exposure time, which gathers light, and trailing, which spoils sharpness. The calculator gives you both figures so you can choose with open eyes instead of guessing in the dark.
A worked example
Take a 24 mm lens on a full-frame camera. Five hundred divided by 24 is 20.8 seconds, so about twenty seconds is your classic maximum. The stricter 300 rule gives 12.5 seconds. Now put the same 24 mm lens on an APS-C camera with a 1.5x crop factor. The crop-adjusted focal length is 36 mm, so the 500 rule gives 13.9 seconds and the 300 rule gives 8.3 seconds. The same lens, the same sky, but a much shorter safe exposure, purely because the smaller sensor magnifies the motion.
At the other end, a 14 mm ultra-wide on full frame gives nearly thirty-six seconds by the 500 rule, which is generous but still limited. That is why ultra-wide lenses dominate Milky Way photography: they allow long exposures that gather faint light without trailing. A 50 mm lens on a Micro Four Thirds camera, with its 2x crop, behaves like a 100 mm lens and allows only five seconds by the 500 rule, which is why normal lenses struggle for night skies on small sensors.
Getting the most from the limit
Within the exposure limit, gather as much light as you can. Open the aperture wide, raise the ISO until noise becomes the problem rather than the solution, and focus carefully on a bright star. Take several frames and stack them in software: stacking adds the light of many short exposures without adding the trailing of one long one, which is the practical way around the rule’s limit. A tracker, which moves the camera with the sky, is the other way around it, and our star tracker calculator covers that case.
Test your own limit once, in your own conditions. Shoot the same scene at several exposures, zoom in on the stars at full size, and find where points become dashes for your camera and your standards. Write that number down for each lens you own. The rule gives you a starting point in seconds; your test gives you a personal rule that you can trust for every night after.
Frequently asked questions
Does the 500 rule work for the Moon? No. The Moon is bright and moves at its own rate, so it needs its own short exposure, typically a fraction of a second. The rule is for stars only.
Why do some photographers use 400 or 600 instead of 500? Because the rule is a convention, not physics. Four hundred is stricter and suits high-resolution cameras, six hundred is looser and suits small prints. The calculator’s 300 figure covers the stricter end honestly.
What if I want trails on purpose? Then ignore the rule entirely and use the star trail calculator instead, which plans exposures of minutes or hours for deliberate arcs.
Use the calculator above before your next night out, note both the 500 and 300 figures for your lens, and remember that the rule is a starting point: sensor pixel density and print size change the true limit, so let your own test shots have the final word.
Related calculators
- Spanish version
- Star Trail Exposure Calculator – Shutter Time for Long Curved Star Trails
- Star Tracker Exposure Calculator – Safe Tracked Exposure by Focal Length