The 500 Rule:
500 Divided By the Focal Length of Your Lens = The Longest Exposure Before star trails start to appear.
For example; let’s say you’re taking a shot with a 14mm lens on a full frame camera. 500 / 14 = 35 seconds "roughly".
Here are a couple of shots taken with a Rokinnon 14mm lens on a full frame camera. At 90 seconds you can see the star trails starting to appear, at 30 seconds the stars are sharp.
I personally would aim for about 20sec with either a higher ISO or larger aperture and leave further brightening of the stars to post processing.
Here's a graphic to help you!
As you may have assumed from my previous statement it is important to always round down rather than up. So instead of 36 seconds i would aim for 30 or even lower at 25 seconds.
This is just a basic introduction to the 500 rule. You could use 400 to reduce your star trails even further but this depends on your cameras capabilities at high ISOs and whether or not you have a fast lens that has a decently large aperture.
For example if you have a lens like the Samyang/Rokinnon 14mm f2.8.
Shooting at f2.8 with a 14mm lens would allow you to use ISO 1600 with a 25sec exposure. This would leave you with sharp stars and minimal ISO noise on even your most recent entry level DSLRs.
I was able to shoot at ISO 1600 with my old Canon 450D and create images i was happy with when i was teaching myself Astro photography.
That all said finding a good post processing technique is important. Maybe that will be my next lesson?!?
Happy Fotoging!
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