Field curvature, also called "Petzval field curvature," is a defect in the lens in which the object of a flat plane is not focused on the image surface. Field curvature measurement is important in lens performance tests. This paper presents an electrowetting liquid lens based on the depth from defocus method for measuring field curvature. This method uses only a pair of defocused images for a patterned flat object, which are captured using the focus tuning function of the electrowetting liquid lens. Image processing for calculating field curvature is carried out using MATLAB. The results of the measurement experiment demonstrated high accuracy of the 12 mu m root mean square error between the captured image surface and fitted curved image surface. There is no need for complex equipment such as lasers, microscopes, and telecentric systems. Because this system has no mechanical movement for focus tuning, it is simple and shows fast measurement time compared with other conventional methods.