Variable focal length optical system

A variable focal length optical system used in combination with an infinity corrected objective lens, the variable focal length optical system comprising:

• an imaging lens configured to condense a parallel beam of light from the objective lens to form an intermediate image;
• a relay system comprising, in sequence from an object side, a first relay lens and a second relay lens, the relay system configured to relay the intermediate image to infinity;
• and
• a lens system of a liquid resonant type which is located between the first relay lens and the second relay lens, the lens system having a refractive index that changes in response to an inputted drive signal, wherein
• a front principal point of the lens system is located to be conjugated with an exit pupil of the objective lens, and
• with fTube denoting a focal length of the imaging lens and fR1 denoting a focal length of the first relay lens, an expression (1) below is satisfied,
• 1.90<fTube/fR1≤2.15 (1).