How long can I expose before star trails appear in my photo?

It depends on focal length, sensor pixel pitch and target declination together, not on any single fixed number — from as little as roughly 6 seconds at 35mm on a 24-megapixel APS-C camera near the celestial equator, to well over 200 seconds at a wide focal length pointed close to the celestial pole. There's no universal answer; the specific combination of your lens, camera and where you're pointing determines it.

The Three Variables That Actually Drive This

Wider focal lengths tolerate longer exposures, since angular star motion covers fewer pixels per second at a wider field of view. Lower-resolution sensors (larger pixel pitch) tolerate longer exposures too, since the same physical trailing is less visible spread across bigger pixels. And targets closer to the celestial pole tolerate dramatically longer exposures than targets near the celestial equator, since they simply move more slowly across the sky. All three stack together, which is why the honest range spans nearly two orders of magnitude across realistic combinations.

Why a Single Rule of Thumb Undersells This

A rule like '500 divided by focal length' captures only one of these three variables directly (it does account for crop factor, but not pixel pitch or declination), which is why it's a reasonable fast estimate but not a precise answer for a specific camera pointed at a specific part of the sky. The NPF rule, with declination included, is the more complete calculation — still an approximation, but one that accounts for all three variables that genuinely determine when trailing becomes visible.

Get a precise answer for your own camera, lens and target in the Astro 500/NPF Rule Calculator.