A sheathless flow cytometry system is disclosed wherein a fluid containing particles of interest is illuminated in the sensing region with a light source. Light resulting from the interaction of the particles with the illumination is received by a lens, and focused toward a field stop having an aperture comprising relatively large end portions and a relatively small center portion. Light deflectors are disposed over the relatively large end portions of the aperture. The system is arranged such that light from particles in focus in the sensing region is focused on the relatively small center portion of the aperture. Peripheral detectors receive light from the light deflectors, and a center light detector receives light passing through the center portion. The detector signals may be used to identify which of the detector signals correspond to particles in focus as they passed through the sensing region.