Piston conversion factor for each mode of each AO system, measured by injecting a modulation on the DM and detecting it with the GRAVITY fringe tracker. The conversion factor is the ratio between the piston measured in μm_rms and the modal modulation in μm_rms. A model with optimal single-mode apodisation confirms the high conversion factors for radial-only focus and spherical modes; the amplitude mismatch arises because the DM gain (see Sect. 2.1) was not yet corrected at the time of the measurement. The small conversion factor for tip-tilt is compatible with a small 2.5% photometric pupil shift. All other modes show little conversion factor, probably within the noise of the measurement.