Zoom photographic optical system

A zoom photographic optical system to be used in an optical apparatus equipped with an objective lens system for forming a final image of a specimen comprising, in order from a rear side of said objective lens system, at least;

• a first lens unit having positive refractive power;
• a second lens unit having negative refractive power;
• and a third lens unit having positive refractive power, wherein said second lens unit and said third lens unit are moved along an optical axis for changing a magnification from a low magnification position to a high magnification position so that an airspace between the first lens unit and the second lens unit is narrower at the high magnification position than that at the low magnification position and an airspace between the second lens unit and the third lens unit is wider at the high magnification position than that at the low magnification position, wherein an intermediate image is formed between the first lens unit and the second lens unit at the low magnification position, and wherein the zoom photographic optical system satisfies the following conditions (1) and (2):
• 1≦FH/FL≦3 (1)
• 3≦MGH/MGL≦20 (2)
• wherein a reference symbol FH represents a focal length of lens units as a whole as counted from the first lens unit to another lens unit disposed just before the intermediate image at the high magnification position, a reference symbol FL designates a focal length of lens units as a whole as counted from the first lens unit to another lens unit disposed just before the intermediate image at the low magnification position, a reference symbol MGH denotes a magnification of lens units as a whole which are disposed between the intermediate image and the final image at the high magnification position, and a reference symbol MGL represents a magnification of lens units as a whole which are disposed between the intermediate image and the final image at the low magnification position.