About this calculator

This site exists to answer one recurring night-photography question clearly: how long can I leave the shutter open before the stars turn into streaks?

It is built and maintained by Marcio. Marcio builds small, single-purpose web tools that do one job well and explain the maths behind them. This astrophotography exposure calculator grew out of a recurring beginner question — 'how long can I leave the shutter open before the stars turn into streaks?' — and the fact that the popular '500 rule' quietly breaks on modern high-resolution sensors.

The calculator gives you two answers for your exact setup. The 500 rule is the quick, traditional estimate — 500 divided by your equivalent focal length. The NPF rule is the sharper, modern one that factors in your aperture and the physical size of a pixel, which matters a great deal on today's high-resolution sensors. An optional declination adjustment accounts for the fact that stars near the pole move more slowly than stars on the celestial equator, so you can expose longer when framing near Polaris.

Everything runs in your browser using JavaScript. No value you enter is uploaded, stored or shared. The tool is provided free for general guidance; it is a starting point for your own testing, not a guarantee — seeing conditions, lens quality and how you view the final image all shift where the practical limit sits.

Spotted a camera preset you'd like added, or a formula detail worth refining? I'd like to hear about it — see the contact page.