The answer is the 500 rule. To avoid star trails, exposures should be less than 500/focal-length-in-135-equivalent. In other words, if he shot this on his 'fav

The answer is the 500 rule. To avoid star trails, exposures should be less than 500/focal-length-in-135-equivalent. In other words, if he shot this on his 'favourite' Rokinon 24mm, an exposure time of 20 seconds is reasonable to avoid all star trails.