A variable focal length (VFL) lens system is provided including a VFL lens, a VFL lens controller, an objective lens, a camera and an optical power monitoring configuration. During a standard workpiece imaging mode, the objective lens transmits workpiece light along an imaging optical path through the VFL lens to the camera, which provides a corresponding workpiece image exposure. During an optical power monitoring mode, the optical power monitoring configuration produces a monitored beam pattern which travels along at least a portion of the imaging optical path through the VFL lens to the camera, which provides a monitoring image exposure. Different monitoring image exposures are acquired at different phase timings of the periodic modulation of the VFL lens, and a dimension of the monitored beam pattern is measured in each monitoring image exposure as related to an optical power of the VFL lens at the corresponding phase timing.