Focal length
Focal length is a basic optical property of a telescope or lens that indicates the distance between the main optical part (e.g. the lens lens) and the focal point, the point where light rays converge to form a sharp image. This distance is usually given in millimetres (e.g. 150 mm, 500 mm, 1200 mm) and determines how large a section of the sky the telescope will show.
A short focal length means a wide field of view and lower magnification, which is useful for large objects such as large nebulae or the Milky Way. Conversely, a longer focal length provides higher magnification and a smaller slice of the sky, making it suitable for viewing smaller objects such as distant galaxies, planetary nebulae, or details of the Moon and planets.
Therefore, the choice of focal length depends on the objects to be observed or photographed: short focal lengths are for wide fields and compositions, long focal lengths for details and small targets.