What’s the longest exposure you can use before you start to get a star trail?That depends on your sensor size and the focal length of your lens! This handy chart gives you the answer using the generally accepted ‘500 rule’. 500/(your focal length in 35mm terms).
This is just a guideline how long to exposure your shots before you get blurry shots due to the earth’s rotation. because sometimes we want star trails and sometimes we don’t.
For custom focal lengths, simply divide 500 by the focal length of your lens. Example, for a 20mm lens: 500 / 20 = 25 seconds on full frame, or 500 / (20*1.6) = 16 seconds on a Canon crop camera or 500 / (20*1.5) = 17 seconds on a Nikon/Sony crop camera.