Astro exposure calculator
The longest shutter speed you can use before stars turn into little streaks - the 500, 400 and 300 rules scaled for your sensor, plus an optional pixel-pitch (NPF-style) estimate.
Lens & sensor
The actual focal length printed on the lens.
Smaller numbers = shorter, safer exposures.
Used to scale the rule by crop factor.
Advanced (optional)
Maximum shutter time
How it works
Because the Earth rotates, every star sweeps across the sky at about 15° per hour. Long enough exposures turn stars into little arcs. The classic rules of thumb tie the safe shutter speed to how wide your lens is - a wider lens spreads the same movement over more sky, so it tolerates longer exposures:
The crop factor matters because a smaller sensor magnifies the same image: a 20 mm lens on a 1.5× APS-C body behaves like a 30 mm lens on full frame, so the safe exposure is 500 ÷ 30 ≈ 16.7 s. Pick a stricter rule (400 or 300) for larger prints, higher-resolution sensors, or wide viewing.
The optional pixel-based estimate uses a simplified NPF-style formula, t = 17 × pixel pitch (µm) ÷ focal length (mm), at declination 0°. It is far stricter than the 500 rule on high-resolution sensors because it tries to keep a star within roughly one pixel - and it is still a simplification: the full NPF rule also accounts for aperture and tolerance. Treat it as a floor, not a guarantee.
Choosing a value in practice
- 500-rule - fine for modest-resolution sensors, small prints and web sharing.
- 400-rule - a safe middle ground for most modern cameras.
- 300-rule or tighter - high-resolution bodies, large prints, or 100% viewing.
- Still seeing trails at your chosen value? Shoot shorter and stack more frames - star movement can never be fully eliminated.
Example
A 20 mm lens on full frame gives 25 s by the 500-rule and 20 s by the 400-rule. The same 20 mm lens on a 1.5× APS-C body gives about 16.7 s (500-rule) - round down to 15 s on the camera. On a modern 45 MP full-frame body, the pixel-based estimate at 4.3 µm suggests roughly 3.7 s, which is why serious wide-field shooters stack instead of chasing long single frames.
Related tools
Pick a dark, core-facing night with the milky way planner, plan a long trailing sequence with the star trail calculator, or balance ISO and aperture with the long exposure calculator.
Frequently asked questions
What is the 500 rule?
The 500 rule says a sharp single-frame star shot needs a shutter speed no longer than 500 divided by your focal length. On a 20 mm full-frame lens that is 500 ÷ 20 = 25 seconds. The 400 rule is a stricter 400 ÷ focal length, and the 300 rule stricter still. They are rough rules of thumb, not physics.
Why do crop-sensor cameras need shorter exposures?
A smaller sensor magnifies the same lens's image, so star movement crosses more pixels in the same time. The calculator multiplies your focal length by the crop factor (a 20 mm lens on a 1.5x APS-C body behaves like 30 mm) before applying the rule.
Is the pixel-based estimate more accurate than the 500 rule?
It is more thoughtful but still simplified. The NPF-style formula used here (17 × pixel pitch ÷ focal length) accounts for how large stars are on your sensor, but it ignores aperture and viewing conditions. On high-resolution sensors it is far stricter than the 500 rule, which is usually the honest answer.
Can I completely avoid star trails?
Not with a fixed camera - stars never stop moving. Longer exposures always trail a little; shorter exposures show more noise. For truly round stars at long exposures use a star tracker, or shoot many short frames and stack them.