Dustproof view finder

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.

We claim: 
     
       1. A dustproof view finder in which an image formed by an objective optical system is erected by an image erecting optical system and is observed by an ocular optical system as an erect image, said dustproof view finder comprising a pair of front and rear optical elements spaced from one another at a predetermined distance to define an aerial space therebetween, and an enclosing device which encloses the aerial space, the enclosed aerial space defining therein an image forming position at which an object image is formed by the objective optical system; wherein the following relationship exists between focal length "fe" of the ocular optical system, . .optical path length.!. .Iadd.distance .Iaddend."d" from the image forming position to a front surface of the front optical element, and refractive index "n" of the front optical element:   |(1000×d)/{n×(fe).sup.2 }|≧2.0.Iadd.. .Iaddend.       
     
     
       2. A dustproof view finder according to claim 1, further comprising a field frame provided on a surface of one of the optical elements located in the closed aerial space. 
     
     
       3. A dustproof view finder according to claim 1, wherein said enclosing device comprises a seal member which is mounted to the optical elements to enclose the aerial space. 
     
     
       4. A dustproof view finder according to claim 3, wherein said seal member is provided with a fitting portion in which the optical elements are fitted. 
     
     
       5. A dustproof view finder according to claim 4, wherein said seal member has an adhesive surface. 
     
     
       6. A dustproof view finder according to claim 5, wherein said seal member is made of an adhesive material. 
     
     
       7. A dustproof view finder according to claim 5, wherein said seal member is made of an adhesive tape. 
     
     
       8. A dustproof view finder including a pair of optical elements spaced from one another, said dustproof view finder comprising a field frame provided between the optical elements, and a seal member which seals said field frame in an . .serial.!. .Iadd.aerial .Iaddend.space therebetween, and an enclosing device which encloses said field frame in a space defined between the optical elements; wherein the following relationship exists between focal length "fe" of the ocular optical system, . .optical path length.!. .Iadd.distance .Iaddend."d" from the space between the optical elements to a front surface of the front optical element, and refractive index "n" of the front optical element:   |(1000×d)/{n×(fe).sup.2 }|≧2.0.       
     
     
       9. A dustproof view finder according to claim 8, wherein one of the optical elements is a field plate which is provided thereon with said field frame. 
     
     
       10. A dustproof view finder according to claim 9, wherein said seal member surrounds the other optical element and said field plate. 
     
     
       11. A dustproof view finder according to claim 10, wherein said seal member has an adhesive surface. 
     
     
       12. A dustproof view finder according to claim 11, wherein said seal member is made of an adhesive material. 
     
     
       13. A dustproof view finder according to claim 8, wherein said seal member is made of an adhesive tape. 
     
     
       14. A dustproof view finder according to claim 10, wherein said seal member is provided with mounting portions to which the other optical element and said field plate are mounted. 
     
     
       15. A dustproof view finder according to claim 8, wherein the space defined between the optical elements defines therein an image forming position at which an object image is formed by the objective optical system. 
     
     
       16. A dustproof view finder according to claim 8, further comprising a field frame provided on a surface of one of the optical elements located in the closed aerial space.