Simultaneous compensation of out-of-plane motion and defocus blur effect in 2D-DIC based on an electrically tunable lens

Two-dimensional digital image correlation (2D-DIC) is indispensable for in-plane deformation measurement owing to its simple configuration and high computational efficiency. However, its measurement accuracy is severely compromised by the defocus blur effect and virtual deformation induced by out-of-plane motion. Existing compensation methods typically address only one of these two issues and tend to fail when severe defocus prevents correlation computation entirely. To address this challenge, this study introduces a compensation method based on an electrically tunable lens (ETL). The mechanism by which the ETL eliminates the influence of out-of-plane displacement and achieves autofocus is first analyzed. Technically, a climbing search algorithm based on DIC measurement strategy rapidly modulates the ETL focal length by targeting stripe spacing, maintaining both image clarity and constant magnification during the image acquisition stage to compensate for defocus blur effect and virtual deformation caused by out-of-plane motion. Multiple experimental validations confirm the effectiveness of this approach. This approach reduces substantial virtual strain to noise levels while simultaneously resolving defocus issues in measurements. This comprehensive compensation fundamentally improves the quality and reliability of the raw speckle images, thereby providing a solid foundation for accurate deformation measurement. The ETL-based method effectively expands the application potential of 2D-DIC for characterizing flexible materials and holds promise for future extension to microscopic deformations.