Into the Stella Valles rode the 600

Quite a while ago I posted a technique on eliminating/reducing star trails in your night shots. And now I have found another, perhaps easier way to make this calculation, at least for safety. This is called the "Rule of 600".

Basically you take your true lens focal length (on a 2/3 sensor you multiply by 1.6, eg 18mm x 1.6 = 28.8mm) and then divide 600 by that focal length .. this will return you the longest exposure that is possible for any given focal length lens, without the stars streaking.

So, using the 18mm Lens, that would return 600/28.8 = 20.833 seconds before trails show up.