This little rule helps us out when we are preparing to take a classic widefield or landscape astronomical shot. Obviously, it remains valid if we are not using a star tracker.
But let's take a step back. We have our nice SLR or mirrorless camera with our wide-angle or, why not, medium-telephoto lens on our tripod; which parameters should we choose?
ISO: obviously choose the highest ISO possible that does not deteriorate the image too much. Usually ISO 1600.
Aperture: easy; unless we have a low-quality lens, we will opt for the widest possible aperture. Usually f/2.8 – f/4.
And the shutter speed?
While the first two parameters seen are roughly the same in most situations, the exposure time varies depending on the type of equipment used to avoid the annoying star trails effect.
The parameters to keep in mind are:
-Sensor crop factor (which we will call cds) is a fixed value (0 for FF, 3 for all other formats)
-Lens focal length (which we will call L)
Therefore, the 500 rule formula will be:
(500/L) – cds
Practical example:
We have a camera with an APSC crop and a 20mm focal length lens:
(500/20) – 3 = 22 seconds
Here is the maximum exposure time within which we will have pinpoint stars with a given set of equipment.
Have fun!