OPTICAL SYSTEM, IMAGE PICKUP APPARATUS, AND IMAGE PICKUP SYSTEM

An optical system includes 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 is fixed during focusing. The first focus lens unit and the second focus lens unit are movable during focusing so that a distance between the first focus lens unit and the second focus lens unit changes. The first lens unit includes a single negative lens disposed closest to the object. A predetermined condition is satisfied.