The 500 Rule
for Astro-Photography

Determining the maximum allowable exposure duration (in seconds)
for a given focal length lens
before the stars begin to blur (move) in the final image

 

capturing The stars! 

One of the challenges of Astro photography is that you must shoot in almost total darkness. This scenario stresses modern cameras to their limits. The standard settings for "Night Photography" are:

Program mode : Manual
ISO : 3200
Aperture : F2.8
exposure duration : 30 seconds
color temp : 3200 (Tungsten)

To be able to use these settings, you need a camera that supports manual settings, adjustable ISO up to 3200, a fast lens (as least F2.8) and you need a lens that can shoot with an exposure duration of 30 seconds without allowing the stars to blur.

The 500 Rule determines how long the exposure (seconds) can be for a given focal length lens (mm) - without allowing the stars to blur. The formula is:  

500
divided by
the Focal Length of the lens (in mm)
equals
maximum Exposure (in seconds)
before the stars start to blur

As it turns out, 16mm lenses are perfect for this equation:

500 / 16mm = 31 second max Exposure

So, in order to achieve the standard settings for night photography, you need a F2.8 16mm lens (or wider - and don't forget about the crop factor on your camera).

Now, you could push the ISO higher, but 3200 is generally the max that can be corrected (in Lightroom CC) for most cameras (Sony excepted). You can also use a faster lens (i.e. F1.4), but these lenses 1) tend to be pricy, 2) are designed for portrait photography, rather than astro-photography (their edges are soft) and 3) their optical sweet spot is generally around F2.8 - F5.6.

Here is the 500 rule calculation for other focal length lenses: 

500 / 14mm = 35 second max Exposure

500 / 16mm = 31 second max Exposure

500 / 20mm = 25 second max Exposure

500 / 24mm = 21 second max Exposure

500 / 50mm = 10 second max Exposure

500 / 100mm = 5 second max Exposure

***

Movement of Stars around the North Star (Polaris) (Click to enlarge)

Note: If you are shooting the North Star (or Southern Cross), the stars closest to the poles are moving relatively slowly (in a tight circle around the pole). But the stars near the 'equator' are moving relatively fast in the night sky (see image).