The invention provides a zoom optical system comprising two fluid lenses (22, 40) and a first fixed lens group (20) of negative optical power, positioned at the object side of the zoom optical system. The first negative lens (20) ensures that a beam in the zoom optical system is relatively narrow, which allows the further elements, in particular the fluid lenses (22, 40) to be smaller in diameter, which is advantageous for their optical quality and for their susceptibility to temperature differences, gravitational influences et cetera.