Optical system, image pickup apparatus, and image pickup system

An image pickup apparatus comprising:

• an optical system comprising a first lens unit having a positive refractive power disposed closest to an object, a first focus lens unit having a positive refractive power disposed on an image side of the first lens unit, and a second focus lens unit having a negative refractive power disposed on the image side of the first focus lens unit, the first lens unit being fixed for focusing, and the first focus lens unit and the second focus lens unit being movable during focusing so that a distance between the first focus lens unit and the second focus lens unit changes;
• and
• an image sensor configured to receive light of an image formed by the optical system,
• wherein the first lens unit includes a single negative lens disposed closest to the object,
• wherein the first lens unit includes an aperture stop and a plurality of positive lenses which are disposed on the image side of the aperture stop, and
• wherein the following inequalities are satisfied:
• −5.0<fp2/fn1<−0.1
• −0.290<n1gt/fn1<−0.095
• where fp2 is a focal length of the first focus lens unit, fn1 is a focal length of the second focus lens unit, and n1gt is a distance on an optical axis from a surface closest to the object of the second focus lens unit to a surface closest to an image plane of the second focus lens unit.