An optical system disclosed by the present invention comprises a diaphragm, a photosensitive chip and a light filter, one side of the diaphragm close to the object side is equipped with a first lens group which can move relative to the diaphragm, a second lens group is arranged between the diaphragm and the light filter, and a third lens group which can move relative to the second lens group is arranged between the second lens group and the light filter. The first lens group comprises a first lens, a second lens, a third lens and a fourth lens, the focal lengths of the first, second and third lenses are all negative, and the focal length of the fourth lens is positive. The second lens group comprises a fifth lens and a sixth lens, the focal length of the fifth lens is positive, and the focal length of the sixth lens is negative. The third lens group comprises a seventh lens and an eighth lens, the focal length of the seventh lens is negative, and the focal length of the eighth lens is positive. The first, second, third and seventh lenses are all meniscus lenses of which the concave surfaces all face the image side. The optical system of the present invention is good in high/low temperature adaptability, high in pixel, low in cost, large in aperture and small in size, and is infrared confocal.