When performing long exposure photography of the night sky, there are limits as to how long the exposure can be before stars appear to trail. Below are two common rules use to determine the maximum exposure length in seconds. The first is easy enough to approximate in your head, while the second is a quite a bit more involved.
\(t \) is the time in seconds of the exposure,
and \( f \) is the focal length of the telescope/lens in mm.
\( t \) is the exposure time in seconds,
\(k\) is the acceptable number of pixels of trailing,
\(N\) is the focal ratio (f-stop),
\(f\) is the focal length (in mm),
\(p\) is the pixel size (in microns),
and \(\delta\) is the declination of the object/area.