Star trails occur when the movement of stars across the night sky causes their images to stretch across multiple pixels in a long exposure photograph. The "500 rule" or "600 rule" can help photographers calculate the maximum shutter speed to avoid star trails based on their camera's focal length. For example, with an APS-C camera using an 11mm lens, the maximum shutter speed to avoid trails would be 28 seconds, calculated by dividing 500 by the full frame equivalent focal length. This rule provides a starting point, but individual results may vary and require testing shutter speeds.

1. What is a star trail? Movement of star captured across several pixels of a given
image leads to the formation of a star trail. This is a really common problem
when doing long exposure night sky photography or astrophotography. Pretty
sure most of us like to star gaze and observe those tiny little twinkling dots in the
sky on a clear day. If our eyes can see it, so why not our DSLR? We want exact
same tiny cute little stars to show up in our DSLR photos too. Once you have
learned how to focus your DSLR lens to infinity, next thing to think about is how to
avoid star trails. This is where 500 rule or 600 rule comes to the rescue. I have
seen a lot of discussions about 500 rule vs 600 rule, I personally have always
been happy with 500 rule only. Sometimes I even go for 400 rule but never 600
rule.
Who invented this rule? I did search but couldn't find anything relevant or
meaningful. Only thing I know is all modern day astrophotography folks love this
rule. I will try to explain the rule with two simple examples:
1) Canon T5i (cropped sensor) camera.
Lens used: Tokina 11-16 F2.8 DX at 11 mm.
ISO: 3200
F-stop: F2.8
Shutter speed:
Maximum shutter speed possible to avoid star trails: 500 divided by full frame
equivalent focal length. So it is 500/(11*1.6) which gives us approx 28 seconds.
1.6 is the crop factor for Canon cropped sensor cameras, you can use 1.5 for
Sony and Nikon cropped sensor cameras.

2. 20 seconds exposure
2) Nikon D610 (full frame sensor) camera.
Lens used: Tokina 16-28 F2.8 FX at 16 mm.
ISO: 3200
F-stop: F2.8
Shutter speed calculation:
Maximum shutter speed possible to avoid star trails: 500 divided by full frame
equivalent focal length. So it is 500/16 which gives us 31.25 seconds. In this
case, I would use 30 to be on safe side. Take a few test shots and check your

3. image by zooming in. If you see every star looking well rounded and crispy, you
have taken an awesome shot. If not, try again with a bit lower shutter speed.
Hope this post would help you to take stunning and crispy star shots without any
surprise star trails. There are certain advanced factors such as sensor size,
image resolution and declination affecting your shutter speed too, but that would
be explained in a new post.
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