Optical system and image pickup apparatus

An optical system with a fixed focal length comprising:

• a front lens unit closest to an object;
• a rear lens unit having a negative refractive power closest to an image plane;
• and
• a middle group having a positive refractive power as a whole, including one or more lens units, and disposed between the front lens unit and the rear lens unit,
• wherein a distance between adjacent lens units changes during focusing,
• wherein the middle group includes three or more positive lenses and one or more negative lenses,
• wherein the rear lens unit consists of two positive lenses and two negative lenses, and
• wherein the following inequality is satisfied:
• -
• 0.4
• ≤
• f
• /
• fr
• <
• 0.
• where f is a focal length of the optical system, fr is a focal length of the rear lens unit.

An image pickup apparatus comprising:

• an optical system with a fixed focal length;
• and
• an image sensor configured to capture an image formed by the optical system,
• wherein the optical system includes:
• a front lens unit closest to an object;
• a rear lens unit having a negative refractive power closest to an image plane;
• and
• a middle group having a positive refractive power as a whole, including one or more lens units, and disposed between the front lens unit and the rear lens unit,
• wherein a distance between adjacent lens units changes during focusing,
• wherein the middle group includes three or more positive lenses and one or more negative lenses,
• wherein the rear lens unit consists of two positive lenses and two negative lenses, and
• wherein the following inequality is satisfied:
• -
• 0.4
• ≤
• f
• /
• fr
• <
• 0.
• where f is a focal length of the optical system, fr is a focal length of the rear lens unit.