The invention relates to an optical system for examining an eye by means of optical coherence tomography. The OCT system is designed in such a way that at least a first and a second state of the optical system can be selectively set by controlling the variable optical unit. In the first state, the OCT measurement beam has a measurement focus at an object distance from the objective, wherein the object distance has a value between 50 millimeters and 400 millimeters. In the second state, the measurement beam has defocusing at the same object distance, wherein the defocusing corresponds to a distance of a virtual or real focus from a position of the object distance that is greater than 100 millimeters.