Star shutter speed calculator
The longest shutter speed that keeps stars as points, from the NPF rule for your sensor, lens and aperture, with the 500 rule beside it.
All tools- NPF rule
- 14 sSet 13 s or shorter
- 500 rule
- 25 sSet 25 s or shorter
- Pixel pitch
- 6 µmSensor width ÷ pixels across
The 500 rule allows 1.8 times longer than NPF here, which will trail on a high-resolution sensor. Use the NPF figure.
How it works
The method behind the numbers.
The sky turns about 15° an hour, so stars drift across the frame during an exposure. The longest shutter speed that still looks like points depends on how long the lens is and how small the pixels are.
500 rule: t = 500 ÷ (f × crop)
NPF: t = (35 × N + 30 × p) ÷ f
p = sensor width (µm) ÷ pixels acrossf is the focal length in mm (the real one for NPF), N the f-number and p the pixel pitch in µm. The 500 rule ignores the sensor beyond its crop factor, which was reasonable for older, lower-resolution cameras. NPF accounts for aperture and pixel size, so it is the better guide for a modern high-resolution sensor.
- Both figures are worst case, for stars near the celestial equator. Stars near the pole star move far less and can take longer exposures.
- Pixel pitch is the sensor width divided by the pixel count across it. Use the image width in pixels from your camera's spec sheet, not the megapixels.
- Treat the results as a ceiling for sharp points at full size. Round down to the next shorter setting, and shorten further for big prints.