A method detects the level of ink in a printhead reservoir by monitoring the power drawn by a thermistor positioned within the printhead reservoir. The active drivable element is positioned so as to not directly support the weight of the gradient coil assembly, which avoids applying strong forces to relatively fragile active drivable elements, such as piezoelectric force transducers. In addition, scanning is not performed on a region from which it is difficult to detect a peak, so that fast and accurate AF may be performed.