An inspection system is disclosed. An optical assembly establishes an optical path between a light source and a detector. The optical assembly has a relatively large amount of longitudinal chromatic aberration, so that light at a first wavelength focuses on one region of a substrate in the optical path, while light at a second wavelength simultaneously focuses on another region of the substrate. The system can operate in a calibration mode to determine one or more wavelengths of light corresponding to regions of interest in the substrate and in an imaging mode to image regions of interest in the substrate.