The invention relates to a method and a sensor device (100) for detecting particles (MP) that are bound to the binding surface (12) of a carrier (11). The sensor device (100) comprises a microscope (50) for imaging bound particles (MP) onto an image sensor (53). In order to increase the spatial resolution of the microscope (50), a displacement unit (60,70,80) is provided that can displace the carrier (11) relative to the image sensor (53). The distance of a bound particle (MP) from the binding surface (12) and/or its lateral displacement in reaction to forces can thus be determine with high accuracy. This allows to discriminate specific bindings of large magnetic particles (MP) that are bound to the binding surface (12) via smaller target particles from nonspecific direct bindings.