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Astro Exposure Calculator
Find the longest shutter speed before stars trail, using the 500 Rule and your lens focal length and sensor crop factor — plus an aperture/ISO settings note.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Astro Exposure Calculator
The "500 Rule": longest shutter speed before stars trail, from your focal length and sensor crop factor.
Aperture and ISO are recorded for your reference — the 500 Rule's shutter-speed limit depends only on focal length and crop factor.
How it works
The 500 Rule estimates the longest shutter speed before stars visibly trail, using the lens's effective focal length (its printed focal length multiplied by the sensor's crop factor):
max shutter seconds = 500 / (focal length mm × crop factor)
- Enter your lens's focal length and pick your camera's sensor type.
- Record your aperture and ISO for reference — they don't change the trailing limit, but they matter for exposure brightness and noise.
- Use the result as your starting shutter speed, then zoom into a test shot at 100% to confirm the stars are still sharp points.
FAQ
What is the 500 Rule?
A widely-used astrophotography guideline for the longest shutter speed you can use before Earth's rotation makes stars visibly trail into streaks: divide 500 by your lens's effective focal length (focal length × sensor crop factor).
Why does crop factor matter, not just the focal length printed on the lens?
A smaller sensor (APS-C, Micro Four Thirds) crops a narrower field of view from the same lens, which is optically equivalent to a longer focal length — so the same 24mm lens trails stars sooner on a 1.5x-crop body than on full-frame. The calculator multiplies focal length by crop factor to get that effective value automatically.
Is the 500 Rule perfectly accurate?
No — it's a practical starting point, not a physical law. High-resolution sensors reveal trailing sooner than the 500 Rule predicts, which is why some photographers use the stricter "NPF Rule" (accounting for pixel pitch) instead. Start here, then check a full-resolution test frame before a real shoot.
Do aperture and ISO affect the maximum shutter speed?
Not for star trailing itself — that's purely a function of focal length, crop factor, and Earth's rotation. Aperture and ISO instead control how bright and how noisy the resulting image is; the calculator records them alongside your shutter speed as a complete settings note.
What focal length should I use for wide Milky Way shots?
Wider lenses (14-24mm) give much longer usable exposure times before trailing — a 14mm full-frame lens allows roughly 35 seconds versus about 21 seconds at 24mm — which is why wide-angle lenses are the standard choice for Milky Way and star-field photography.
How we compare
| Feature | Online Tool Store | Memorized rule of thumb | Dedicated astrophotography planning app |
|---|---|---|---|
| Accounts for sensor crop factor automatically | Yes | Easy to forget | Usually yes |
| Instant, in the browser, no install | Yes | N/A | Often app-only |
| Records aperture/ISO alongside the result | Yes | Manual notes | Sometimes |
| Free, no sign-up | Yes | N/A | Often paid |
For a quick, correct starting shutter speed before a night shoot, this beats doing the crop-factor math by hand in the dark.