Waveguide and glass capillary optics have been used to study morphological variations across single as-spun poly(p-phenylene benzobisoxazole) (PBO) fibres. The waveguide provides a beam with a submicrometre dimension perpendicular to the fibre axis. The glass capillary optics provide a beam of a few micrometres in diameter. By directly comparing results obtained with both optics it is possible to assess the influence of beam size on experimental observations. The study demonstrates that submicrometre beams are invaluable for detailed morphological studies across single fibres. The greater morphological averaging inherent to data collected using glass capillary optics reduces the variation between skin and core measurements, as well as obscuring other important features. Crystalline domain orientation, (200) crystal plane spacings and reflection intensities can be determined from data generated using a submicrometre beam. All parameters are found to vary significantly across the diameter of single PBO fibres. In terms of orientation, both skin and core regions exhibit a lower degree of crystalline domain orientation than an intermediate zone. In terms of (200) crystal plane spacing, the observed variations suggest residual stresses within the fibre structure following manufacture.