Method of monitoring an image capture system (1) comprising a sensor (C) comprising a plurality of photosensitive elements (Z1, Z2, Zn) and an optical device (L) for focusing the light emitted from a scene towards the sensor. This method comprises the obtaining (100) of respective responses of certain at least of the photosensitive elements (E1, E1′, P1, P2) of the sensor to an exposure of the image capture system to any scene (S), followed by a determination (200) of at least one deviation (Δ) between at least one quantity (G) deduced from the responses obtained and at least one reference quantity (Gref). These steps may be followed by an estimation (300) of an optical effect of the image capture system (1) on the basis of said deviation (Δ) determined and optionally by an implementation of an action able to at least partially compensate (400) for the estimated optical defect.