Enrico Marega Interactive photography course
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Astro­photography: the 600 and 500 rules

The Earth turns, and the sky with it: in a long exposure the stars become trails. To keep them as points there’s a maximum exposure time, which depends on the focal length and the sensor.

1Simulator

How long the exposure can last

Choose the focal length, sensor and exposure time. The magnifier shows the stars at 100%: dots or dashes?

The 600 rule

1Simulator The maximum time in seconds is roughly 600 divided by the equivalent focal length. With a 50 mm on full frame: 600 / 50 = 12 seconds. With the same lens on APS-C (crop 1.5): 600 / 75 = 8 seconds. It’s a rule that dates back to the days of film.

1Simulator With today’s high-resolution sensors, the stars already look a little elongated if you zoom in a lot: that’s why many people use the more conservative 500 rule, or the NPF rule, which is even more precise. If you want trails instead, you need exposures of many minutes or lots of photos to stack together.

Key points

  • Maximum time for pinpoint stars ≈ 600 / (focal length × crop) seconds; with the 500 rule you play it safer.
  • Short focal length and wide aperture: for example 14–24 mm, f/2.8 or wider, ISO 1600–6400.
  • For star trails: very long exposures or lots of photos stacked together, pointing towards the Pole Star to get circles.

Try it yourself

On a moonless night, far from lights, with a tripod: wide-angle, aperture wide open, ISO 3200, manual focus on a bright star. Work out the time with the 600 rule, then take the shot again with the 500 rule and once more with twice the time: zoom in and compare the stars.

Post the result and tag me @enrico.marega, or send it to me by DM: I’d love to see how you tackled it.

Quick check

50 mm on full frame. What is the maximum time according to the 600 rule?
Same lens, but on APS-C. The maximum time…