Two deflection-angle prisms were used to finely focalize the fundus of an eye when the fundus was observed in the dark. A slit mark was projected onto the fundus. Light transmited in two different directions through the slit mark because of the bending of the deflection-angle prisms. The slit mark was broken into two slit marks. When the fundus was in focus

these two broken slit marks would be in a line. These two slit marks would be taken apart when the fundus was out of focus. The separated distance and direction of these two slit marks were depended on the state of the defocus. Experiment showed that it was simple and had high precision when one judged focus states basing upon whether these two slit marks were in a line.