Overwatch Soldier looking through Combine Binoculars, shortly before being killed.
The Combine Binoculars are dark colored devices with a shimmering blue lens with a green sight set into a small, compact set. The device can be fixed on the edge of a window, its tripod planted into the wall beneath. On the other side of the set seems to be some sort of rack with different lenses, possibly to be changed depending on the use.
After killing the Overwatch Soldiers stationed in the small house, Freeman himself can observe N.L.O. through the binoculars. There he witnesses Odessa Cubbage and the G-Man chatting at the top floor of N.L.O.'s main building, then going back inside together, and Rebels walking around, notably one of them pushing a dead Antlion off the cliff.