A dynamic focusing system (DFS) for an optical device for observing an object. The dynamic focusing system (DFS) has an objective housing (OBH) including a circumferential housing part (CIH) and a light-transmitting window (LTW); a chamber (ICH) filled with an immersion medium having a refractive index substantially equal to the refractive index of the object to be observed; a movable immersed objective (MIO) configured to move along at least one direction so as to vary the focusing depth and being at least partially immersed in the immersion medium. The immersion medium is confined in the chamber (ICH) by a deformable sealing member (DSM) attached to the movable immersed objective (MIO) and to the objective housing (OBH). The deformable sealing member (DSM) is configured to ensure that the volume of the chamber (ICH) remains substantially constant upon displacement of the movable immersed objective (MIO) relative to the objective housing (OBH).