a three - dimensional shape measurement system produces three - dimensional information of an object , such as an eye fundus , by directing light onto the object , receiving the light reflected from the object , photoelectrically converting the received light and processing the electric signal . the system includes laser beam control circuitry for focal point shifting in the direction of the optical axis , and a pair of detectors for detecting variation in the focal position of the light reflected from the object and deriving information respecting the shape of the object in the direction of the optical axis . signal processing circuitry drives the laser beam control circuitry to produce a focused condition and adjusts the output signals of the detectors to a prescribed state .