Abstract: An optical device and the prism module thereof are provided. The prism module includes a first prism, a second prism, and a third prism. The second prism is disposed beside the first prism. The third prism is adhered to the second prism. First light enters the first prism, is reflected plural times in the first prism, enters the second prism, and is emitted from the second prism. Second light enters the second prism, is reflected plural times in the second prism, and is emitted from the second prism. Third light sequentially passes through the third prism and the second prism, enters the first prism, is reflected plural times in the first prism, and is emitted from the first prism.

Abstract: An optical system includes an objective lens module, an image inverting module, and an eyepiece module. The objective lens module includes a first lens group, a second lens group, a third lens group, a fourth lens group, and a fifth lens group. The optical system satisfies at least one of the following conditions: 0.45?LG4D/LG1D?0.8; 0.015 mm?1?1/fG3?0.045 mm?1; 0.045 mm?1?|1/fG4|?0.07 mm?1; 0.35?|fG4/fG3|?0.75; 0.15?fG1/f?1.6; wherein LG4D is an effective optical diameter of the fourth lens group, LG1D is an effective optical diameter of the first lens group, fG1 is an effective focal length of the first lens group, fG3 is an effective focal length of the third lens group, fG4 is an effective focal length of the fourth lens group, and f is an effective focal length of the objective lens module.