1.binoculars是复数形式的名词，但表达的是单数概念，若表示数量，须用单位词pair。如“一副双筒望远镜”是a pair of binoculars。
2.binoculars用作主语时，谓语动词应该用复数形式。

I watched the birds through my binoculars.我用双筒望远镜观鸟。
He watched the play through his binoculars.他用双筒望远镜看戏。
The binoculars are in that leather case.双筒望远镜在那皮箱里。