After the 500 rule did disappoint me in avoiding star trailing on my Milky Way photos I went in search of a better solution. The NPF rule was the answer. No star trails and but of what cost? Reducing the shutter speed will ad some limitations on how much light the sensor will recover.
Calculate the NPF Rule (English scroll down)
This rule, NPF, has been developed by Frédéric Michaud from Le Havre Astronomical Society (SAH) for the use of photographers and amateur astronomers. No commercial use is allowed without prior consent. Any development using this formula shall give credit to the author and the SAH.
The Results of 500 RULE vs NPF RULE
If you have different camera/lens you have to redo the test for your self. Let’s take in count what I used to photograph. Canon R6 with Laowa 15mm f/2.
Only looking the startrails the 500 rule (left) leaves a startrails, not much but enouf viseble for big prints.
From the NPF rule to the 500 rule do we lose much light?
Yes, we do… almost one stop of light. BUT this is in my configuration. Make the calculations for your camera and lens combination.