A binocular incorporates a focusing and diopter correction mechanism having a common shaft, whereby the diopter adjustment is performed by rotation of the shaft and the focusing operation is performed by axial movement of the shaft. The lens-moving portions of the focusing/diopter adjustment mechanism are in a cavity having an access hole, in which the shaft enters the access hole through a water-tight seal to control the lens-moving portions of the adjustment mechanism. The binocular also incorporates a strap attaching mechanism having a spring-loaded clip and groove, wherein a flanged pin provided to a strap end is pushed past the clip into the groove, and the clip returns under the bias of the spring to hold the flanged pin in the groove. Also included is an eyecup adjustment mechanism having two positions defined by click-stops, wherein the eyecups are not rotatable, but are longitudinally slidable relative to corresponding support barrels.