A variable focal length lens system includes a tunable acoustic gradient (TAG) lens; a TAG lens controller; and processor(s) configured to: (a) operate the TAG lens controller to drive a periodic modulation of the TAG lens optical power at a TAG lens resonant frequency, using a first amplitude driving signal that provides a first focal Z range extending between peak focus distances Z1max+ and Z1max−; and expose a first image using a first exposure increment approximately at the timing of Z1max+ or Z1max−; (b) adjust the TAG lens controller to drive the periodic modulation using a second amplitude driving signal that provides a second focal Z range extending between peak focus distances Z2max+ and Z2max−; and expose a second image using a second exposure increment approximately at the timing of Z2max+ or Z2max−; and (c) perform processing that includes determining focus metric values for the first and second images.