The great part about the 500 Rule is that it can also work in reverse for the shooter that is looking to capture star trails. For those of you that don’t shoot with full frame cameras it is necessary to take this “Crop Factor” into account when performing the calculation. I have provided standard crop factors for different camera models below. For example a full frame 35mm camera shooting at 16mm can capture the night sky for a maximum of 31 seconds prior to showing any results of “Star Trails” in the picture. Once the exposure time exceeds 31 seconds, trails or star movement will start to show up in the shot. The best way to clearly exhibit this relationship is to go outside and experiment, there are a million different acceptable combinations it’s just a matter of finding what works for you and your camera. My Free Star Photography Tutorial also provides info on this topic.