Low light and Surveillance
For surveillance at a distance, a high magnification is useful, around 20x will allow you to see detail at a considerable distance.
Larger lenses let in more light and allow the binocular to work in lower light conditions. A 20x80 is an ideal all-round observation binocular but this needs mounting on a solid tripod.
For handheld, low light surveillance, select 8x56 or 9x63.