Oblique-viewing objective optical system and endoscope for oblique viewing using the same

An oblique-viewing objective optical system, comprising in order from an object side:

• a front-side lens group which includes a negative lens;
• an optical path converting element;
• an aperture stop;
• and
• a rear-side lens group having a positive refractive power,
• wherein:
• the rear-side lens group includes a positive lens and a cemented lens having a positive refractive power,
• the cemented lens includes in order from the object side, a positive lens and a negative lens, and
• the following conditional expressions (1), (2), (3), (4), and (5) are satisfied:
• −2.0<fF/f<−1.3 (1),
• 1.7<fR/f<2.7 (2),
• 0.63<|fF/fR|<0.88 (3),
• 2.4<D1/f<4.4 (4), and
• 1.1<D2/f<1.7 (5),
• where:
• fF denotes a focal length of the front-side lens group,
• fR denotes a focal length of the rear-side lens group,
• f denotes a focal length of the overall oblique-viewing objective optical system,
• D1 denotes an air-conversion length from an image-side surface of the negative lens in the front-side lens group up to the aperture stop, and
• D2 denotes an air-conversion length from an image-side surface of the negative lens of the cemented lens in the rear-side lens group up to an image plane.