The Rule of 500

The rule of 500 is super old and predates high resolution sensors (like APS-C sensors).

focus length in mm / 500 = exposure time in seconds

Using the rule of 500, my Sony a6000 using an 85mm lens would mean I would see star trails if I kept the shutter open (aka the exposure) for longer than 5.8 seconds. And also a full frame A7 III the same would see the same even though its pixels are much larger.

Obviously that is just wrong.

If you just want to sanity check use the rule of 300:

focus length in mm / 300 = exposure time in seconds

This is a better rule of thumb these days.

While there are apps that do this, I believe this is the actual source is:


And you can maybe understand this better translated to English:


The true rule of 500 is actually from our guy down under:

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