500 Rule & NPF Calculator
How long can you expose before the stars turn into streaks? Here you get both answers: the classic 500 rule and the more precise NPF formula.
500 Rule
—
simple guideline
NPF Rule
—
precise (recommended)
💡 Use the NPF value and round down to the nearest available shutter speed. If you need more light: open the aperture first, then raise ISO – don't expose longer.
Why is there a limit at all?
The Earth rotates – and with it, stars appear to drift across the sky at roughly 15 arcseconds per second. On a fixed tripod (without tracking), a long exposure smears every star into a small streak. The longer the focal length, the more it magnifies this movement – which is why Milky Way shots are almost always taken with wide-angle lenses.
500 Rule: simple but outdated
The rule of thumb 500 ÷ (focal length × crop factor) dates from the film era. It's easy to remember, but ignores sensor resolution: modern cameras with 40+ megapixels show star trails much sooner than the rule allows. At 100% crop or large prints, images taken at the 500-rule limit are often visibly blurred.
NPF Rule: precise
The NPF formula (by Frédéric Michaud) factors in aperture (N), pixel pitch (P) and focal length (F): t = (35 × aperture + 30 × pixel pitch) ÷ focal length. It typically gives about half the 500-rule value – the price for truly point-like stars. If that's too short: stack multiple exposures or use a star tracker to lift the limit entirely.
When is it worth it?
The best exposure time is worthless without a dark sky: check with the Milky Way Calculator when the galactic core is visible, and with the Moon Calendar that no bright moon is interfering. What a night like that actually feels like is in my blog post on astrophotography at Melchsee-Frutt.
Keen on a shoot under the open sky? Write to me – I photograph weddings, maternity, families, couples, portraits, babies and events in Lucerne and across Switzerland.
Did you enjoy this tool?
All tools here are free, ad-free and run entirely in your browser. If this calculator helped you, I'd appreciate a coffee. ☕
Scan PayPal & buy me a coffee