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The 500 Rule

A balancing act is needed when you are trying to capture a starry sky with a standard camera, lens and tripod. You need to have a long enough exposure time to have the stars as visible as possible, but not too long or your image will have star trails. The 500 rule can be your guide in finding a balance between having the stars be pinpoints or having star trails.

The 500 rule states that when using a certain focal length, divide it into 500, and the result is the maximum exposure in seconds that you can expose. Keep in mind that these are full frame equivalent focal lengths, so don't forget to do your APS or Micro 4/3 conversions if that is your sensor format.

Here's an example that I did years ago. I was using a 17mm lens on a full frame camera.

500 / 17 = 29.4 seconds

This was a 30 second exposure and the stars are pinpoints.

Let's look at another example of a 30 second exposure but this time with slightly more magnification with a 24mm lens:

The 500 rule says with a 24mm lens that we can expose for 500 / 24 = 21 seconds. We exposed almost 50% longer than we should have. Let's have a closer look at the stars:

You can see that the stars in the green circle (to the right) are slightly deformed due to motion. The more we push the exposure length, the less the stars will look like pinpoints.

While a great guide, the 500 rule can also be bent a little bit, depending on what you are hoping to achieve. If you are trying to get star trails, then expose for as long as it makes sense for your gear and your scene.


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