An observation optical system of the present invention comprises an image erect optical system and an eyepiece optical system for observing an object image. The eyepiece optical system comprises a positive single lens having double convex surfaces and a negative meniscus lens having a concave surface directed toward eye point side, and the following condition is satisfied: 0

 
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> Image taking apparatus capable of optically and electrically changing magnification of taken images

> Cam mechanism of a photographing lens

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