An image sensor operable to vary the output spatial resolution according to a received light level while maintaining a desired signal-to-noise ratio. An electronic shutter of a CCD image sensor 23 capable of outputting an image sensing signal in a progressive scan mode is controlled by a timing signal generator 28, with the field cycle according to the standard television system used as a basic cycle, to cause the CCD image sensor 23 to output an image sensing signal in the progressive scan mode. The first and the second latch mechanisms are connected to the driving mechanisms. The identification of the software and selection of a corresponding installation includes identification of a version of the software. In case of a wrong answer within the standard time at a first presentation of optotype, reexamination at the same visual acuity is executed only once.