500 Rule Calculator
Work out the longest exposure you can shoot before stars trail, for any focal length and sensor.
Enter a focal length — results update as you type.
- Max exposure (500 rule)
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- Nearest shutter speed
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- Effective focal length
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- 300 rule (stricter)
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- 200 rule (pixel-peeping)
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That is longer than the 30 s most cameras allow — switch to bulb mode with a remote or intervalometer.
How it works
The 500 rule is the classic astrophotography shortcut for the longest exposure you can shoot
on a fixed tripod before the Earth's rotation turns stars into short streaks. The formula used
here is t = 500 ÷ (focal length × crop factor), where the focal length is the real
number engraved on the lens and the crop factor converts it to a 35 mm-equivalent. A 24 mm lens
on full frame gives 500 ÷ 24 ≈ 20.8 seconds; the same lens on a 1.6× Canon APS-C body gives
500 ÷ 38.4 ≈ 13.0 seconds.
The 300 and 200 variants are the same formula with a smaller numerator. They exist because the
500 rule was written for film and low-megapixel sensors — on a modern 45 MP body the trailing
the 500 rule allows is clearly visible when you zoom in. The NPF rule shown below is the more
rigorous alternative, calculated here in its simplified form
t = (35 × N + 30 × p) ÷ f, with N the f-number, p the pixel pitch in microns and f
the actual focal length in millimetres. It gives shorter, safer times for fast, wide lenses on
dense sensors. Pixel pitch = sensor width in mm ÷ horizontal pixel count × 1000, so a 36 mm
wide, 6000 px wide 24 MP full-frame sensor gives 6.0 µm.
These are rules of thumb, not physics. Stars near the celestial equator trail fastest; stars near Polaris trail far more slowly, so you can often exceed the number when shooting north. Always zoom to 100 % on the back of the camera and check.
Everything is arithmetic done in your browser with plain JavaScript. Nothing is uploaded, there is no sign-up, no ads and no tracking of what you type.
Frequently asked questions
What is the 500 rule in astrophotography?
The 500 rule estimates the longest tripod exposure you can shoot before the Earth's rotation turns stars into streaks. Divide 500 by the 35 mm-equivalent focal length: t = 500 ÷ (focal length × crop factor). A 24 mm lens on full frame gives about 20.8 seconds.
How does the crop factor change the answer?
A smaller sensor magnifies the same star motion, so the safe exposure gets shorter. Multiply the real focal length by the crop factor first — 1.5× for most APS-C bodies, 1.6× for Canon APS-C, 2.0× for Micro Four Thirds. A 24 mm lens on a 1.6× body behaves like 38.4 mm, so 500 ÷ 38.4 ≈ 13 seconds.
Should I use the 500 rule or the NPF rule?
The 500 rule was written for film and low-resolution sensors, so on a modern high-megapixel camera it allows trailing that is obvious at 100 %. The NPF rule, calculated here as t = (35 × f-number + 30 × pixel pitch) ÷ focal length, accounts for aperture and pixel density and gives shorter, sharper results. Use the 500 rule for a quick field estimate and NPF when you plan to pixel-peep.