What this rule basically tells you is: the longer the focal length, the shorter you must expose your photo, if you want to avoid trailing stars.
What this rule basically tells you is: the longer the focal length, the shorter you must expose your photo, if you want to avoid trailing stars. But this rule is not absolute and certainly not right for everyone. If you take your photos primarily to be showcased on the web, then you’ll be fine using it. But if you also like to print your photos large – 18 inch or more – then you might want to reconsider.