An optical system is provided for creating a mosaic image of a large field of view through a microscope at fast refresh rates of about 25 Hz with a high resolution that is free of blurring or aberrations. The optical system includes an objective lens assembly (20), an iris (30), one or more scanning mirrors (40) for high-speed scanning, one or more imaging lenses and irises (50, 60, 80), and a high-speed imaging device (70) arranged in that order from an object. The optical system also includes a mechanism for processing and constructing scanned and captured images into a mosaic image.