Abstract: An optical system for a binocular is provided with an objective optical system, an erecting system and an eyepiece. The objective optical system includes a first lens having positive refractive power, a second lens having negative refractive power and a third aspherical lens having positive refractive power. The first and second lenses are cemented to each other. The third lens is capable of moving in a direction orthogonal to an optical axis of the objective optical system. The objective optical system satisfies the following relationship:-0.30<.phi..sub.c /.phi..sub.13 <0where, .phi..sub.c is a refractive power of the cemented surface of the first and second lenses, and .phi..sub.13 is a composite power of the first, second, and third lenses.

Abstract: A dustproof view finder in which an image formed by an objective optical system is made erect by an image erecting optical system and is observed through an ocular optical system as an erect image, the optical system including a pair of front and rear optical elements spaced from one another at a predetermined distance to define an aerial space therebetween, and a seal member which encloses the aerial space.