Low Light & Surveillance
For surveillance at 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 20×80 is an ideal all-round observation binocular but this needs mounting on a solid tripod.
Another option for long range observation is a spotting scope with a zoom eyepiece mounted on a tripod, this will allow you to locate a subject easily at low magnification and then increase the magnification to enhance the detail.
For handheld, low light surveillance, select 8×56 or 10×56 binocular.