Fixed-tripod night sky

Milky Way exposure, measured in pixels

Compare the full NPF calculation with the old 300 and 500 rules. The result shows what each shutter time means on your sensor instead of presenting one magic number.

Camera, lens and field Sensor width, mm Image width, pixelsUse the uncropped file width from the camera. Actual focal length, mmDo not multiply by crop factor. Aperture, f-number Lowest declination in the frame, degreesMilky Way core is about −29°. Use 0° when unsure for a conservative speed estimate. Visible trailing toleranceThis is an explicit NPF tolerance, not a change to the camera.

Exposure comparison

Pixel pitch—derived from sensor and image width

Full NPF range——

Simplified NPF—ignores declination and tolerance

300 rule——

500 rule——

Starting recipeBracket one frameinspect stars at full resolution before the sequence

Calculating…

Formula, source and limits

The full and simplified NPF formulae are credited to Frédéric Michaud and the Société Astronomique du Havre. Read the author’s formula and conditions. This free, non-commercial tool uses the formula with credit; commercial reuse requires the author’s permission.

The full result uses k × (16.9N + 0.10f + 13.7p) / (f cos δ). The movement readout independently projects the sidereal rate through focal length and pixel pitch. Seeing, focus, aberrations, resampling and where the frame is inspected can dominate sub-pixel differences, so the result is a starting range, not a guarantee.

Turn one exposure into a complete star-trail sequence →