The present invention is a block ambient light using the average diameter of the aperture ring is reflected in the inspection of Optometrists and relates to the average diameter of the ring in disciplined optometric apparatus and method of auto-focusing auto-focusing is made so geomangi that compares the two signals after the detection of the respective light detector. ; The invention of the eye to be examined installing the reflected beam splitter mirror for reflecting the ring at the same time for transmitting the image of a ring shape on the cornea and the eye examination, that the image transmitted through the beam splitter optometric ring mirror is an optometric hand ring that is reflected from the beam splitter mirror images taken with a light detector After the stop, and to record in a separate photodetector, in the two images for the eye examination ring light detector detects the average diameter after detecting the difference of the two compared average diameter difference between the average diameter ring optometric will have to be stopped is within a preset tolerance value, and the focusing error value is outside the optometrist performing auto focusing by moving the ring in a direction in which the detection value detected geomangi during operation by a difference between the average diameter of the aperture.
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