A lens assembly exhibiting a certain fixed relative aperture is comprised of a physical acoustic lens having a given physical aperture and a fixed relatively short focal length serially spaced from an electronic acoustic lens having any selected one of a plurality of different relatively long focal lengths and simultaneously controlling the physical aperture of the electronic acoustic lens such that the relative aperture of the assembly as a whole remains fixed regardless of the selected one of the long focal lengths. Such a lens assembly is suitable for use in a pulse-echo ultrasonic-imaging system for imaging a deep structure.