Abstract: A real image mode optical system includes an objective lens forming an image of an object and an eyepiece for observing the image and is provided with a variable focal length mirror which dispenses with any polarizing plate so that a diopter change can be corrected due to the change of its optical power. Accordingly, the real image mode optical system has many advantages important for practical use that the reduction of the amount of light is slight, the characteristics are little changed by gravity, the number of parts is small with low manufacturing costs, the strain of the lens and the like of the optical system is not viewed, aberrations are little produced, and the effective aperture of the entire optical system can be set at a minimum.

Abstract: The present invention provides a variable magnification viewfinder optical system, comprising from the object side: objective lens group of a positive refractive power, including a first lens unit of a negative refractive power and a second lens unit of a positive refractive power, said first and second lens units being shiftable along the optical axis of the optical system for changing the magnification of the optical system; condenser lens group of a positive refractive power; and eyepiece lens group of a negative refractive power; wherein the objective lens group forms a real image of an object and the real image can be observed through the eyepiece lens group; and wherein the optical system fulfills the following conditions;-0.09/mm<1/f.sub.1 <-0.03/mm0.04/mm<1/f.sub.2 <0.11/mm-0.1/mm<fe/(f.sub.1 .multidot.f.sub.2)<-0.035/mmwherein: f.sub.1 represents focal length of the first lens unit of the objective lens group in millimeters; f.sub.