THE 500 RULE
Getting the maximum shutter speed duration gets tricky because everything is moving, even if we don’t feel it.
The Earth rotates, our solar system moves, the constellations move, the galaxy moves. The numbers are very high. So I went very seriously (unnecessarily complicating things) to work on how fast were the visible stars in relation to my camera on earth. Although irrelevant, I even took into account my distance from the equator to find my relative rotational speed around earth’s axis. However I had to face two major issues: not having even remotely the privileged mind of Srinivasa Ramanujan (The Man Who Knew Infinity) to solve the equations, and the limitations of film speed at that time. I discovered that shorter focal lengths were better for this, but all my images of stars had trails, even when using night surveillance film.
It was not until the advent of digital SLRs and high ISO image quality that shooting into a starry night sky with round stars became possible for us, without a motorized computer controlled telescope. By the time I got interested and had a DSLR, all about vector theory was completely forgotten so I just went to our local planetarium and asked how they photographed stars. The answer was: “Amateur astronomers developed empirically a simple yet effective rule: The 600 Rule, where 600 is mm-seconds. When you divide 600 mm-secs over the focal length of your lens in mm, you get the maximum seconds time length to have well-rounded stars, without trails.” Ah!
Later it was found out that 600 was too large a number for prints above 8x10 inches and so the 600 Rule became the 500 Rule.
Below a shutter speeds table:
And you may want be more conservative and make your own 450 Rule, provided you have a good very high ISO performance camera.
How forgiving is this 500 Rule? Not much. My own image at Death Valley still looks acceptable on an 8X10 print, but not so hot on an 11x14 or larger.
As the table shows, the mistake was exceeding the 500 Rule shooting at 30 seconds with 24mm focal length. For that focal length I should have used 20 seconds, or zoom back to 16mm focal length.
Just remember that your only variable for exposure will be ISO. Aperture is the widest open and the shutter speed is fixed, governed by the 500 Rule.
In closing, I would like to show you the stunning photographs made by two of my dearest friends and mentors, Ernesto Santos (esantos) and Larry Anderson (mnbuilder49). These images they made while traveling together to Needles, Custer State Park, in the Black Hills, South Dakota.
You have a question regarding your astrophotography? No problem - just ask in our Astrophotography forum.
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