An efficient OCT data collection and processing method for obtaining a high-axial-resolution image with explicit ranging over an extended depth is described. The method includes collecting a first dataset at a transverse location of the sample. The first dataset comprises spectra of a bandwidth (Δ；k1) sampled at a spectral sampling interval (dk1). A second dataset comprising spectra of a bandwidth (Δ；k2) sampled at a spectral sampling interval (dk2) is collected. The bandwidth Δ；k2 is less than Δ；k1 and spectral sampling interval dk2 is less than dk1. The first and the second datasets are processed to generate at least one A-scan with an axial resolution higher than the axial resolution corresponding to the bandwidth Δ；k2 and a depth range larger than the depth range provided by sampling interval dk1.
