NPF rule (pin-sharp stars)—
500 rule (classic)—
Pixel pitch · crop—
How it works#
500 rule: t = 500 ÷ (f × crop). NPF (simplified): t = (35·N + 30·p) ÷ f, where p is the pixel pitch in µm. NPF is stricter on high-resolution sensors where small trails become visible.
Quick reference (f/2.8)#
| Focal | 500 rule FF | 500 rule APS-C | NPF FF 24 MP | NPF FF 45 MP | NPF APS-C 24 MP |
|---|---|---|---|---|---|
| 14 mm | 36 s | 24 s | 19.9 s | 16.4 s | 15.4 s |
| 16 mm | 31 s | 21 s | 17.4 s | 14.4 s | 13.4 s |
| 20 mm | 25 s | 17 s | 13.9 s | 11.5 s | 10.8 s |
| 24 mm | 21 s | 14 s | 11.6 s | 9.6 s | 9.0 s |
| 28 mm | 18 s | 12 s | 9.9 s | 8.2 s | 7.7 s |
| 35 mm | 14 s | 10 s | 7.9 s | 6.6 s | 6.1 s |
| 50 mm | 10 s | 7 s | 5.6 s | 4.6 s | 4.3 s |
Last updated October 03, 2026 · How we calculate · Sources