If you are photographing the sky, you need to take into account the rotation of the Earth. Some sort of tracking device is required otherwise you will see trailing or motion in your images.
If your camera is on a fixed tripod without tracking, it's easy to calculate the maximum time exposure you can make before the Earth's motion will become apparent in your image.
The maximum time is related to the focal length of the image. The longer the focal length the higher the magnification, the shorter the time.
The maximum time (T) in seconds is given by the following formulae
For 35mm film cameras and full frame DSLR's :
T(s) = 750 ÷ focal length (mm)
For DX sized sensors (15x23) found in most entry and mid-level DSLR's :
T(s) = 500 ÷ focal length (mm)
For smaller sensors found in high end compacts and Olympus DSLR's
T(s) = 300 ÷ focal length (mm)
|Focal length||FULL FRAME CAMERAS||MOST DSLR'S||OLYMPUS DSLR|