Focus detection device and method

The invention provides a focus detection device and method, an imaging unit converges an illumination light beam to a detected workpiece and generates a focusing light beam to enter a detection unit, and a first focusing lens or a pinhole diaphragm in the detection unit is translated along the direction of an optical axis to change the relative position of the first focusing lens and the pinhole diaphragm so as to detect the focus of the detected workpiece. The translation position of the first focusing lens or the pinhole diaphragm is recorded through a scale, the function relation between the defocusing amount of the measured workpiece and the reading of the scale is established according to the optical power which is obtained by a detector and penetrates through the pinhole diaphragm, and then the defocusing position of the measured workpiece is obtained. The focus detection device provided by the invention can accurately judge the out-of-focus positions of the detected workpieces with different reflectivi