An optical system is provided implementing a field correction apparatus. The system includes a rotating support driven by a motor. A plurality of focussing lenses are circumferentially spaced about the axis of rotation of the support. As the support rotates, each focussing lens successively passes over a record medium in an arcuate path. The support is also provided with a pyramidal distributing mirror having a number of reflecting surfaces. Each reflecting surface of the disturbing mirror is associated with one of the focussing lenses. The pyramidal distributing mirror is mounted on the center of the support and receives an optical beam on one of its reflecting surfaces. The optical beam is conveyed to the reflecting surface associated with the focussing lens passing over the record medium in a plane that includes the axis of rotation of the support. The optical beam is also directed to the reflecting surface at an angle to the axis of rotation. A field correction lens receives the reflected beam from the reflecting surface and refracts the optical beam so that the optical beam after emerging from the field correction lens follows a path substantially coincident with a radial line extending from the axis of rotation of the support to the optical axis of the focussing lens during the entire pass of the focussing lens over the record medium. A method of aligning an optical beam to become incident on the optical axis of a focussing lens is also provided.