Max shutter time for sharp stars — NPF, 500, and 300 rules.
Pick your camera to use its real pixel size.
NPF rule (accurate)
—
pixel-size aware
500 rule
—
classic, forgiving
300 rule
—
safer, high-res
600 rule
—
forgiving, film-era
🌌 How to use it: set your shutter to (or just under), open the aperture fully, then raise ISO until the histogram is ⅓–½ from the left. Check at 100% zoom — if stars are elongated, shorten by 20%.