Variable-Magnification synthetic aperture Digital holographic microscopy integrated with dual electrically tunable lenses and oblique illumination

Digital holographic microscopy (DHM) incorporating electrically tunable lenses (ETLs) enables non-mechanical continuous zoom, yet its performance is fundamentally constrained by two coupled mechanisms—NA restriction induced by beam truncation and curvature-dependent dynamic aberrations—which collectively induce magnification-dependent resolution loss and image degradation. This study presents a variable-magnification oblique-illumination synthetic-aperture DHM (VM-OI-SA-DHM) that integrates a dual-ETL zoom imaging module with a multi-directional illumination module based on a spatial light modulator and a Fresnel biprism. Synchronized dual-ETL dynamic zooming enables continuous magnification from 9.3 × to 44.7 × under a fixed object–image conjugate plane, while multi-angle sub-aperture spectrum stitching compensates for the resulting NA loss and aberrations, recovering isotropic lateral resolution consistently superior to conventional ETL-based DHM across the zoom range. Quantitative evaluations demonstrate a stable 0.87 μm lateral resolution from 13.8 × to 44.7×, a 1.78-fold improvement over variable-magnification normal-incidence DHM. The synthetic equivalent NA increases by 75% (from 0.20 to 0.35 at 10.3 × ) and by 57% (from 0.28 to 0.44 at 22.1 × and 38.0 × ), accompanied by an 8.21-fold reduction in background phase noise and a structural distinguishability index of 0.9786. Validation against white-light interferometry yields a phase reconstruction relative error of 4.39%. Biological imaging of skeletal muscle and ascaris sections confirms high-fidelity phase reconstruction. By compensating for these ETL-induced limitations, this work establishes a robust framework for cross-scale quantitative phase imaging and lays the groundwork for future magnification-scalable resolution enhancement.