During long night sky exposures the starts appear to be moving across the sky due to the earth’s rotation. This causes what are most commonly referred to as “star trails”. Sometimes this is a desired effect but most often we try to eliminate them where possible. Especially when photographing the Milky Way. For this reason, The 500 Rule was created. It’s a simple formula that helps eliminate star trails in your images. The formula is as follows:
Divide 500 by the focal length of your lens. The result is your exposure (in seconds). Anything beyond that will result in trails. Anything below that will result in fairly rounded stars. So, if you are shooting with a 20mm lens, then your longest exposure should be 25 seconds.