High-speed variable-focus optical system for extended depth of field

A high-speed camera is essential for observing dynamic phenomena. Such a camera needs a bright lens with large aperture due to the short exposure time. Since the depth of field (DOF) becomes shallow when using a bright lens, there is a demand for a high-speed camera with an extended depth of field. One commonly used technique to extend the DOF is to synthesize images captured at various focus positions. However, this method cannot be applied to a high-speed camera because no conventional mechanism can shift the focus position quickly enough. In this paper, we propose a new high-speed variable-focus optical system for extending the DOF. The proposed optical system is based on a high-speed variable-focus liquid lens with millisecond-order response time that was developed by our group. We show that our proposed system results in better light intensity than the method involving decreasing the diameter of the aperture stop. Furthermore, our experiment demonstrated that a 1000-fps movie with extended DOF was successfully acquired using the high-speed variable-focus optical system in combination with the image synthesizing technique.