Calcolatore Tempo Esposizione Stelle — Formula
## Maximum Exposure Time for Point Stars
The NPF rule estimates how long you can expose before Earth's rotation causes star trailing.
### NPF Formula
**t = (35N + 30p) / (f * cos(dec))**
- *N* = lens f-number
- *p* = pixel size in micrometres
- *f* = focal length in mm
- *dec* = declination of the target
Stars near the celestial equator trail fastest; those near the poles trail slowest.
The NPF rule estimates how long you can expose before Earth's rotation causes star trailing.
### NPF Formula
**t = (35N + 30p) / (f * cos(dec))**
- *N* = lens f-number
- *p* = pixel size in micrometres
- *f* = focal length in mm
- *dec* = declination of the target
Stars near the celestial equator trail fastest; those near the poles trail slowest.
Esempio Risolto
200 mm f/4 lens, 3.75 um pixels, shooting the celestial equator.
- t = (35*4 + 30*3.75) / (200 * cos(0))
- t = (140 + 112.5) / 200
- t = 252.5 / 200
- t = 1.26 seconds