The invention relates to an optical system for a laser therapy instrument, suitable for laser surgery of the cornea and the crystalline lens of a human patient's eye. Such a system comprises a femtosecond laser as a radiation source (2) for the therapeutic laser radiation and an objective (8) from which the therapeutic laser radiation exits and is directed and focussed on to or into the eye. The objective (8) is configurable to direct the focus separately to a region comprising the cornea in a first configuration, and to a region comprising the crystalline lens and the lens capsule in a second configuration, thereby limiting an object-side Z scanning range to two separate regions lying between the front surface of the cornea and the rear surface of the lens.
