How to avoid Star Trails with the '500 Rule'
Due to the rotation of the earth it appears as though the stars are moving through the sky in long exposures. Star trails can be a desired effect when done for much longer exposures, but in this case we want points of light to represent how we see the stars with our eyes. To achieve points of light you can use a simple rule, it's often called the '500 rule'.
500 Rule: 500 divided by the focal length of your lens = the longest exposure before stars start to trail or blur (in seconds)
For example; let's say your taking a shot with a 24mm lens on a full frame camera. 500 / 24 = 21 seconds, which you can round to 20 seconds.
The example above was taken with a 14mm lens on a full frame camera, at 90 seconds you can see the blur of the stars, but at 30 seconds the stars are nice sharp points of light.
You may see this as the 600 rule, I personally do not believe this is the best approach as you will have a small amount of trail using 600. If you never intend to print your images very large then you can use this number to capture a bit more light and nobody will know any better. Smaller prints or web sizes will not show this small trail, but large prints will. This is your choice, but we do not know what the future holds so you should consider starting with 500, and with newer sensors like the Sony a7S you could with a higher ISO and instead use a 400 rule to ensure your prints have no trails.
Math and sleepless nights do not mix (I've learned the hard way) so here is a handy cheat sheet you can print out and keep with you.
|Seconds Before Stars Blur|
|Focal Length||Full Frame||Nikon 1.5 Crop||Canon 1.6 Crop|
Note that I did not round these numbers, you do not have to use the exact number, and I would round down, for example for 18 seconds would round to 15 rather than 20. It is possible to use the exact number if you have an intervalometer; simply set your shutter to BULB and set your intervalometer with these settings: Delay-0, Long-18", Intvl-1", N-1. This will take 1 shot that is 18 seconds long.
Follow this simple formula and you will be well on your way to creating images like the one below.
There has been some discussion on various social networks as to why those with higher megapixel cameras can see star trails when using the 500 rule. The problem comes when viewing raw files at specific zoom percentages. What you see is dependent on the amount of megapixels. In the example below I zoomed in to 400% to show an extreme example of this. The D700 appears to have no trails and the 5D MKII appears to have not insignificant trails (both were taken at 14mm, 30 seconds).
Here's where things get interesting. I opened both files in Photoshop and re-sized them to 24x36. With the playing field leveled we now have a 1/16" star trail in both files!
What's the takeaway? First off, megapixels have no effect on the 500 rule. When you print, the star trails will be the same size. What it really comes down to is how big you are going to print and what the viewing distance is. This will determine how long your trails can be, sorry I don't have an answer for this. Can you perceive a 1/16" trail on a 24x36 print? I highly doubt it. A 60x90 print will have 3/32" trails, this is getting to be a decent size. But, how close should you be viewing a 60x90 print? not the same distance as a 24x36. Ultimately it's up to you, the artist to decide what is acceptable. I'm content with this length of trail and will be sticking with the 500 rule for now.