Photography: 500 rule

When taking an untracked photo of the night sky using a camera on a tripod, this rule tells you how long you can expose before the stars begin to trail. You take the number 500 and divide by the focal length of your lens. By far the simpler of the two popular rules for astrophotography is the 500 rule. It recommends that your shutter speed is equal to 500 √∑ Equivalent Focal Length. So, if your full-frame equivalent focal length is 20mm, the 500 rule would suggest that you use a shutter speed of 500 √∑ 20 = 25 seconds.