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21 May 2022

500 Rule

The 500-Rule states that to obtain a clear image of stars without trails, take the number 500 and divided it by the focal length to get your exposure time. For example, a 20 mm lens would call for an exposure of about 25 seconds and theoretically, still obtain the stars without trails.

The 500 Rule

What is the 500 rule? The wider your lens, the longer your shutter speed can be before the stars start to move.

The equation divides 500 by the focal length of your lens. For example, with a 50mm focal length, you’ll have 10 seconds before the stars move (500 divided by 50 equals 10). With a 24mm focal length, you’ll have 40 seconds before the stars move.

Basically, the shutter speed limit changes based on your focal length.Here is a rough starting point to help practice:

  • On a zoom lens: ISO = 6400, aperture = f/2.8, shutter speed = 15 seconds
  • On a prime lens: ISO = 3200, aperture = f/1.4, shutter speed = 8 seconds

The earth rotates at around its axis 1,000 miles per hour and hurtles around the Sun at about 70,000 miles per hour, yet the stars in the distance seem to creep idly by every night. When deciding on what maximum exposure time should be used these Astro bodies can be challenging for those trying it for the first time. For astronomical photos of the Moon’s surface, usually, the Looney-11 rule is a great starting point (f/11 and shutter speed to ISO setting). Similar to the Sunny-16 Rule, it allows for sharp well-exposed images without a light meter. Star trails can be a desired effect, but for images of the stars-as-dots, the 500-Rule should be followed.

Provided you have the right equipment, it mostly comes down to tweaking your camera settings. A wide fast lens, such as the Tamron 15-30mm f/2.8, are fantastic for capturing images sharp clear images. Another useful tip is to avoid light pollution from major metropolitan areas. Sites like Dark Site Finder help you find the area closest to you that has the minimal amount of pollution, though sometimes it is hours away it can be worth it especially if you have never witnessed the full splendor that is the Milky Way.