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I'm a newbie on everything that surrounds astrophotography. I've been reading about the 500 rule and how to avoid star trails. My question is why you divide by the focal length? I mean how the focal length affects the light coming into the sensor and thus having star trails. I know its not an accurate science but I'm just curious.

Thank you :)

marked as duplicate by mattdm, Hueco, inkista, xiota, Olivier Aug 19 at 20:39

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Because longer focal length is greater magnification increasing the size of image objects, including the length of of star trail blur.

The original 500 rule was for 35 mm film. However, it does not take other sensor size into account, but that is also a trail length factor. See my calculator about the 500 rule,

Think of it this way: Let's say I place my index finder 3 inches from your eyes and then move my finder back and forth. Now let's say I'm one mile away and move my finder back and forth the exact distance. At 3" my finger moves a lot from your perspective, but at one mile, you may not even notice that my finger is moving at all. So it is with longer lenses. The wide-angle lens is like when I was standing one mile away, while telephoto lenses bring the stars closer and therefore their movement is much easily noticed.

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