Left: change in multiplicative bias (top) and additive bias (bottom) as a function of Δ, the error in PSF ellipticity. Right: idem, but when the size of the PSF is changed isotropically. The relative change in size is quantified by , computed using the PSF quadrupole moments. In both cases, the multiplicative and additive biases increase linearly with the PSF error, but we note the different scales: The multiplicative bias is more sensitive to size errors, whereas the additive bias is most sensitive to errors in the PSF ellipticity.