The 500 rule — what shutter speed keeps stars sharp?

Updated Nov 1, 2026

The Earth turns, so stars slowly move across the sky. Expose too long and they stretch from points into short lines. The question is how long is too long — and that depends on your lens and camera.

The 500 rule

The classic rule of thumb:

Longest shutter time (seconds) = 500 ÷ (focal length × crop factor)

The crop factor is 1 for full-frame, about 1.5 for most APS-C cameras (1.6 for Canon APS-C), and 2 for Micro Four Thirds.

Why the 500 rule is only a starting point

The rule was made for film and older, lower-resolution cameras. On today's high-megapixel sensors, you can see small trails at the times it gives, especially when you zoom in.

• Many photographers use 400 or even 300 instead of 500 for sharper stars.
• Stars near the celestial equator (roughly south in the Northern Hemisphere, where the Milky Way core is) move fastest and trail first.
• Stars near the pole move slowly and can take longer exposures.

A more precise method: the NPF rule

The NPF rule also takes the aperture and the pixel size of your sensor into account. It usually gives shorter times than the 500 rule, and it is the better choice when you want stars pin-sharp at full resolution. It is tedious to work out by hand, which is why most people use a calculator or an app.

Milky Way Finder's camera settings helper does this for you: pick smartphone, full-frame, APS-C and more to get the longest shutter time that keeps stars sharp, along with step-by-step settings to start with.

Putting it into practice

1. Work out your starting time with the rule (or the app).
2. Take a test shot and zoom in to 100% on stars near the edge.
3. See short lines? Shorten the shutter by a few seconds and raise ISO to compensate.
4. Once the stars are points, lock that in for the night.