Objective optical system and endoscope using the same

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

• a positive first group;
• a negative second group;
• a negative third group;
• and
• a positive fourth group,
• wherein:
• the second group and the third group move together toward an image side to perform focusing from an object point at a long distance to an object point at a close distance,
• the lens furthest to the object side, from among lenses of the positive first group, is a flat concave lens having a concave surface directed toward the image side, and
• the following conditional expression (2) is satisfied, and at least one conditional expression of the following conditional expressions (3), (4), and (5) is satisfied:
• 0.2<(t34f−t34n)/F<2 (2),
• 0.1<(t12n−t12f)/(t34f−t34n)<2.2 (3),
• −1<fG1−1/F<−2 (4),
• 1.8<fG1−2/F<3.5 (5),
• where:
• t34f is a distance between the third group and the fourth group at a time of focusing to the object point at the long distance,
• t34n is a distance between the third group and the fourth group at a time of focusing to the object point at the close distance,
• F is a focal length of the overall objective optical system at the time of focusing to the object point at the long distance,
• t12n is a distance between the first group and the second group at the time of focusing to the object point at the close distance,
• t12f is a distance between the first group and the second group at the time of focusing to the object point at the long distance,
• fG1−1 is a focal length of a lens nearest to the object in the first group, and
• fG1−2 is a focal length of a positive sub-lens group subsequent to the lens nearest to the object in the first group.