a, Schematics of sample geometry of 1-μm-diameter fluorescent beads in an air-filled capillary tube. b, Lateral and axial (along red dotted line) images of beads without and with AO (phase modulation). Post-objective power: 0.13 mW. Digital gains were applied to No AO images to increase visibility. c,d, 3P signal improvement (AO/No AO) and axial full width at half maximum (FWHM) of a representative bead (white arrowhead in b) as a function of the iteration #, respectively. e, Corrective wavefront, unwrapped (modulo-2π) for visualization purposes. Scale bar, 5 μm. Microscope objective: NA 1.05 25×. Representative results from 2 imaging sessions.