Optical system and image pickup apparatus

An image pickup apparatus comprising:

• an optical system;
• an image pickup element configured to receive an image formed by the optical system,
• wherein the optical system includes, in order from an object side to an image side:
• a first lens unit having a positive refractive power;
• a first focus lens unit having a positive refractive power and configured to move toward the object side for focusing from infinity to a closest object distance;
• and
• a second focus lens unit having a negative refractive power and configured to move toward the image side for focusing from infinity to the closest object distance,
• wherein an interval between each pair of adjacent lens units is changed during focusing, and
• wherein conditional expressions:
• −5.00<DA/fA<−0.18;
• D/f<1.00, and
• 0.735≤f1/f<1.20,
• are satisfied where fA is a focal length of the first focus lens unit, DA is a movement amount of the first focus lens unit for focusing from infinity to the closest object distance, “f” is a focal length of the optical system, D is a total lens length of the optical system, and f1 is a focal length of the first lens unit.