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.