Abstract: Quantitative information collection is important for scholars using questionnaires, since the number of administered questionnaires in a major survey is usually quite large. To develop an automatic and vast input method for questionnaires by computer vision, the authors adopt the perspective projection model in the image processing. This study presents a method to calibrate the camera used in the framework of this questionnaire input system. The distortions of the questionnaire image are derived therein so that the system is able to locate all checkboxes accurately. The computer software is coded with a visual programming language and is developed to extract the interiors of all checkboxes by seeking their edges. The experiment is designed, by reducing the errors produced by the recognized centers of some designated checkboxes, to determine the optimal values of various focal lengths to the image plane.