Ok, it's not secret at all, but is sure is asked about a ton!
One of the most important things when it comes to good looking landscape astrophotography shots is crisp round stars. This is achieved by knowing how long you can keep your shutter open without having the stars move (trail) in the image.
I recently posted an image on G+ and a question was asked in the replies. The question was regarding something called the “600 Rule” or the “Rule of 600”.
It isn’t some secret code among astrophotographers….. Oh wait…. Forget what I’ve said until now, there is no rule of 600, it doesn’t exist, enter the Men in Black…….
Seriously, there isn’t some great secret, there is however a bit of math, which some may find as scary as a visit from the “Men in Black”.
Ok, here is the meat and potatoes of the 600 rule.
600 / Real Focal Length = Maximum exposure before trailing occurs.
Confused yet? I thought so……
Lets use a real world example. I shoot with a Nikon D7000 which uses a 1.5 crop sensor (Canon uses 1.6). We’ll use an 10mm lens to keep the numbers easy.
To figure out exposure length we take the lens focal length of 10mm and multiply it by the crop factor of our camera we are using mine so that will make it 10x1.5 = 15
The effective focal length is 15mm
600 / 15 = 40 (seconds)