500 Rule Calculator: Max Exposure Before Star Trails

When the camera is on a fixed tripod, the Earth keeps turning, so leave the shutter open too long and the stars smear into short streaks. This works out your longest safe exposure two ways: the quick 500 rule, and the sharper NPF rule that also accounts for your sensor's pixel size and aperture. The catch with the 500 rule: it was written for full-frame film, so on a crop sensor or a high-resolution camera it is often too generous and stars still trail.

The two rules answer the same question differently. The 500 rule just divides 500 by the full-frame-equivalent focal length, so a 24mm lens on full frame gives 500 ÷ 24, about 20 seconds. It is easy to remember and fine for small prints and wide lenses, but it ignores how tightly your sensor records detail. The NPF rule factors in the pixel pitch and aperture, so on a modern high-resolution body it usually returns a shorter, stricter time that keeps stars as crisp points when you zoom in to 100 percent.

Not sure of your pixel pitch? It is the physical width of one pixel on the sensor in microns. You can look it up for your camera model, or estimate it: divide the sensor width in millimetres by the horizontal pixel count, then multiply by 1000. A full-frame 24-megapixel body is around 5.9 microns; a 45-megapixel one is around 4.3. Leave the field blank and you will still get the 500 rule figure.

For genuinely round stars at any focal length, a star tracker beats both rules: it turns the camera with the sky so you can expose for minutes, not seconds, and gather far more light.