Magnification-varying optical device

A magnification-varying optical device comprising an objective lens, an eyepiece, and a focal length-varying lens placed between the objective lens and the eyepiece on an optical axis passing through the objective lens and the eyepiece, the focal length-varying lens capable of moving along the optical axis, the device further comprising:

• an adjusting lens for adjusting optical properties when the focal length-varying lens is in an area to provide a magnification within a predetermined range;
• an adjusting lens-supporting mechanism for moving the adjusting lens between a first location on the optical axis and a second location off the optical axis, and for non-movably holding the adjusting lens at either of the first location and the second location;
• and
• an interlocking mechanism for interlocking the adjusting lens-supporting mechanism with a magnification-varying mechanism for adjusting a position of the focal length-varying lens, wherein the interlocking mechanism moves the adjusting lens-supporting mechanism so that the adjusting lens is on the optical axis when the focal length-varying lens is in the area to provide a magnification within the predetermined range, and the adjusting lens is off the optical axis when the focal length-varying lens is in an area to provide a magnification outside the predetermined range.