Abstract: The invention relates to an optical system (1) comprising a solid lens barrel (2) for optical elements (3), wherein the lens barrel (2) extends along an optical axis (100) of the optical system (100), the lens barrel (2) holding a plurality of optical elements (3), the plurality comprising a lens system (5), comprising at least one solid lens (53), and a first lens (6) having an adjustable focal length, wherein the first lens (6) comprises a container (62) filled with a transparent fluid, wherein the container (62) comprises an elastically deformable and transparent membrane (61) facing a transparent bottom portion (67) of the container (62), wherein the lens barrel (2) has a first and a second opening (22, 22?) facing along the optical axis (100), wherein the first and/or the second opening (22, 22?) is configured to receive the lens system (5), wherein the lens barrel (2) has a first slot (24) extending perpendicular to the optical axis (100), wherein the first lens (6) is inserted through the first slot (2