A formula by Frédéric Michaud (Société astronomique de France) for the longest exposure without star trails on a fixed tripod: t (s) = (35 × N + 30 × p) ÷ f, where N is the f-number, p the pixel size in µm and f the focal length in mm. At f/2.8 with 4 µm pixels and a 24 mm lens it gives about 9 s. It is more reliable than the old 500 rule with today’s sensors.
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Related terms
- 500 rule
An old, simple rule: longest exposure (s) = 500 ÷ full-frame equivalent focal length (mm), so about 21 s at 24 mm. Devised in the film era, it leaves slightly elongated stars on modern high-resolution sensors. The NPF rule is preferred.
- Star trails
The streaks stars leave on a photo taken from a fixed tripod, because of the Earth’s rotation. Unwanted in a sharp image, they become an artistic effect when hours of exposures are stacked: the stars then draw circles around the celestial pole.
- Exposure
How long the sensor collects light for one frame. For deep sky, take many exposures of 30 s to a few minutes and stack them; for planets, exposures of a few milliseconds freeze the seeing. Without tracking, exposure is limited by the NPF rule.
- Star tracker
A small motorised equatorial mount designed for a camera and lens. It follows the stars and allows exposures of several minutes without trails with a 14 to 200 mm lens. It fits in a bag and needs careful polar alignment.
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