If you're one of the many people who do not believe in rules for photography, you can skip this. : )

I came across this last night - the "500 Rule for taking pictures of the stars and the Milky Way.

"The 500 rule for a full frame camera requires you to set your camera to ISO 3200 or 6400, Aperture to f/2.8 (or as wide as possible) and your shutter speed to 500 divided by the focal length of your camera. For example, if you are shooting with a 50mm lens, your shutter speed would be 10 seconds (500 / 50 = 10)."

If you are shooting with a 24mm lens, your shutter speed would be 21 seconds (500 / 24 = 21 approximately)." I think that should be "...focal length of your lens."

"The crop factor of a Canon crop sensor camera is 1.6 so you need to account for that in your equation. Then it becomes this:

500 / focal length / 1.6. For example, with a 50mm lens it is 500 / 50 / 1.6 = 6 seconds (approximately).

The crop factor for a Nikon crop sensor camera is 1.5 so you account for that like this:

500 / focal length / 1.5. For example, with a 50mm lens, it is 500 / 50 / 1.5 = 7 seconds (approximately)."

I didn't include a link, but Google will find articles for you.