Two cosmological probes, the type Ia Supernovae (SNIa) and the cosmic shear, are being used to study dark energy and dark matter using wide field imagers on large optical telescopes. While the SNIa measurement gives the best measurement of dark energy to date, the cosmic shear, detected in 2000, is still waiting for a precise measurement. Both observations are currently the object of extensive efforts aiming at reducing their error budget. In this talk, I will address the observation, the interpretation and the removal of an important instrumental signature, the ‘brighter-fatter effect’ which affect both the photometry of stars and the shape measurement of galaxies.