Astro Exposure Calculator
The longest shutter speed you can use before stars start to trail, for an untracked Micro Four Thirds camera on a tripod. Uses the NPF rule with your camera's real pixel pitch, which is far more accurate on high-resolution M43 sensors than the old 500 rule.
The number printed on the lens, not the full-frame equivalent.
The aperture you will shoot wide open at.
NPF rule — max shutter
pin-point stars at 100%
NPF ×2 (accurate mode)
slight trailing, more light
Old 500 rule
too long on M43 — for reference
This uses the simplified NPF rule, which assumes a worst-case star position near the celestial equator. It is deliberately conservative, so it can give a slightly shorter time than apps like PhotoPills that factor in the exact declination of your target. Treat it as the safe upper limit.
Pixel pitch by camera
The NPF rule depends on pixel pitch. Higher-resolution bodies have smaller pixels, so stars trail across a pixel sooner and the safe shutter speed gets shorter.
Why the 500 rule fails on Micro Four Thirds
The 500 rule says your maximum shutter speed in seconds is 500 divided by the full-frame equivalent focal length. On M43 that means 500 ÷ (focal × 2). It is quick to remember, but it was written for film and low-resolution sensors. It only asks one question: how fast does a star move across the frame? It ignores how small your pixels are.
That matters because a 20MP or 25MP M43 sensor packs a lot of pixels into a 17.3 × 13mm area, so each pixel is tiny — around 3.3μm. A star only has to drift a fraction of that distance to smear across two pixels and look like a short streak at 100%. The 500 rule lets the star move far too far before it warns you, so photos that look fine on the back screen reveal trailing when you zoom in.
What the NPF rule does differently
The NPF rule, popularised by the French astrophotography group SAHAVRE and built into apps like PhotoPills, factors in three things instead of one: aperture (N), pixel pitch (P) and focal length (F). The formula is:
shutter (s) = (35 × aperture + 30 × pixel pitch in μm) ÷ focal length in mm
Because pixel pitch is part of the equation, the result is tailored to your exact body. A wider aperture also shortens the safe time slightly, because a brighter, sharper point of light shows trailing more readily. The focal length here is the real number printed on the lens, not the full-frame equivalent — the NPF formula already works in true focal length.
This calculator uses the simplified NPF formula, which assumes a worst-case star position near the celestial equator (where stars move fastest). The full version of the rule, used by apps like PhotoPills, also factors in the declination of your specific target and so can allow a slightly longer exposure. Treating the simplified result as a safe upper limit means stars stay tight no matter where you point.
Which number should I use?
- NPF rule is the strict setting. Stars stay as tight points even when you pixel-peep. Use it when you plan to print large or crop in.
- NPF ×2 (accurate mode) doubles the time. You accept a tiny amount of trailing that is invisible at normal viewing sizes, in exchange for a full stop more light. This is what most people actually shoot at for the Milky Way.
- 500 rule is shown only so you can see how much longer — and how much looser — it is than NPF on your camera.
Getting a usable Milky Way exposure on M43
Shutter speed is only one corner of the exposure. M43's smaller sensor collects less light than full frame, so you will lean harder on aperture and ISO. A practical starting point on a moonless night under dark skies: the widest aperture your lens allows (f/1.7 to f/2.8 on the fast wide M43 primes), the NPF ×2 shutter speed from this tool, and ISO 1600 to 3200. Take a test frame, check the histogram, and raise ISO if the foreground is too dark. Shooting several frames and stacking them later is the most effective way to beat M43 noise without a star tracker.
If you own an OM System or Olympus body, Live Composite and Starry Sky AF make this far easier — Starry Sky AF nails focus on the stars automatically, which is the step most people get wrong in the dark.