The invention relates to a compound objective lens for a Scanning Electron Microscope having a conventional magnetic lens excited by a first lens coil, an immersion magnetic lens excited by a second lens coil, and an immersion electrostatic lens excited by the voltage difference between the sample and the electrostatic lens electrode. For a predetermined excitation of the lens, the electron beam can be focused on the sample using combinations of excitations of the two lens coils. More BSE information can be obtained when the detector distinguishes between BSE's (202) that strike the detector close to the axis and BSE's (204) that strike the detector further removed from the axis.By tuning the ratio of the excitation of the two lens coils, the distance from the axis that the BSE's impinge on the detector can be changed, and the compound lens can be used as an energy selective detector.