6XC

500 Rule Calculator (Star Photography)

Maximum shutter speed before stars trail — classic 500 rule plus the more accurate NPF rule for modern high-resolution sensors.

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The 500 Rule and the NPF Rule

Earth's rotation smears stars into trails on long exposures. The classic 500 rule says: max shutter = 500 ÷ (crop factor × focal length) — a 20 mm lens on full frame allows about 25 s. On modern high-megapixel sensors that is often too generous, so the NPF rule refines it using aperture and pixel pitch: t = (35 × f-number + 30 × pixel pitch in µm) ÷ focal length. This calculator shows both — treat NPF as the safe value and 500 as the upper bound.

Frequently Asked Questions

Why do my stars still trail at the 500-rule time?

The rule dates from film and low-res sensors. On a 45 MP body, pixels are small enough to record motion far sooner — the NPF result (often half the 500-rule time) is the realistic limit when viewing at full size.

What settings work for the Milky Way?

A typical starting point: widest lens you own, f/1.8-f/2.8, ISO 3200-6400, shutter at the NPF value. Shoot RAW and focus manually on a bright star using live view at maximum zoom.

How can I expose longer than these limits?

A star tracker cancels Earth's rotation, allowing minutes-long exposures. Alternatively, stack many short exposures with software like Sequator or DeepSkyStacker for lower noise without trails.

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