An optical apparatus for adjusting the position and aperture of a pinhole and a method using the same are provided. A light beam is provided. The light beam is focused on an object and reacts with the object to form a signal beam. The signal beam is focused and projected on a liquid-crystal switch. The projection position of the signal beam on the liquid-crystal switch is determined, and the transparence of the liquid-crystal switch at the projection position is adjusted to form a transparent area. The signal beam passes through the transparent area and reaches a light detecting unit to form a detecting signal. The aperture of the transparent area is adjusted according to the intensity of the detecting signal. The liquid-crystal switch is driven to move, so that the position of the transparent area in the moving direction of the signal beam is adjusted.