Astrophotography Exposure

A longer focal length magnifies everything in the frame, including the apparent speed at which stars drift due to Earth's rotation. The same angular rotation of the sky covers proportionally more pixels on a telephoto lens than a wide-angle lens, so star trails become visible sooner -- the NPF rule's exposure time shrinks roughly in proportion to how much the focal length grows, which is why wide-angle Milky Way shots tolerate exposures of 15-25 seconds while a 500mm lens on the same sky might be limited to just a couple of seconds.

A star's trail has to cross a meaningful fraction of a pixel's width before it becomes visibly elongated rather than a sharp point. A sensor with larger, more widely spaced photosites (a bigger pixel pitch in micrometers) can tolerate more physical star movement across the sensor before that threshold is crossed than a high-resolution sensor with tightly packed small pixels, so cameras with larger pixels generally support proportionally longer untracked exposures at the same focal length.

Earth's rotation sweeps every star around the celestial pole at the same angular rate per unit time, but a star's APPARENT motion across the sky (in right-ascension terms relevant to trailing) scales with the cosine of its declination. A star sitting near the celestial equator (declination near 0) traces the largest apparent arc, while a star near the pole (declination near +/-90) traces a much smaller one in the same time, so it takes proportionally longer for its trail to become visible.

The 500 rule is a simple mental-math shortcut (500 divided by focal length in millimeters) that ignores sensor pixel size and aperture entirely, so it tends to overestimate the safe exposure time on today's high-resolution sensors, where trails become visible sooner than the 500 rule predicts. The NPF rule instead factors in aperture, focal length, and actual pixel pitch, making it noticeably more conservative and more accurate on modern cameras -- which is why this calculator shows both, with the NPF figure as the primary, more trustworthy result.

No -- the NPF rule targets the threshold where trailing becomes noticeable at typical viewing sizes, not a guarantee of zero star movement. Atmospheric seeing, focus accuracy, tripod stability, and how much you crop or enlarge the final image all affect whether trailing is visible in practice, and any exposure right at the calculated limit will show some trailing under close inspection or heavy cropping -- shooting somewhat under the calculated maximum leaves margin for these factors.