Should I use the 300 rule or the 500 rule for astrophotography?

At 20mm on full frame, the 300 rule (300 ÷ focal length) allows roughly 15 seconds; the 500 rule allows roughly 25 seconds at the identical setup — the 300 rule is simply a stricter version of the same simple calculation, trading exposure time for a safer margin against visible trailing.

Why a Stricter Divisor Exists at All

500 was calibrated for a fairly generous standard of acceptable sharpness — reasonable for typical viewing sizes but genuinely borderline on a modern high-resolution sensor viewed at full zoom. 300 emerged as a photographer-adopted correction for exactly that gap, well before the more rigorous, camera-specific NPF rule existed to properly account for pixel pitch — a rough, resolution-blind way of building in extra safety margin rather than a formally derived alternative.

Why the NPF Rule Made Both Somewhat Obsolete

Both 300 and 500 apply the identical fixed divisor regardless of your camera's actual resolution, which means 300 is unnecessarily conservative on a lower-resolution body and can still be too generous on a very high-resolution one — the NPF rule solves this properly by using your camera's real pixel pitch instead of guessing at a one-size-fits-all safety margin. If you have your pixel pitch handy, the NPF figure is simply the more accurate choice over either fixed-divisor rule.

Compare all three approaches for your own camera in the Astro 500/NPF Rule Calculator, pre-filled at 20mm.