The invention relates to a
medium wave infrared 30 times continuous zooming optical
system without a rear fixed group. The optical
system consists of a vari-focal objective lens and a
repeater group from an object side to an image side; the vari-focal objective lens consists of a front fixed group lens, a
variable time group and a compensating group, the focal powers of the three groups of lenses are distributed into a positive structure, a negative structure and a positive structure in sequence, and the
zoom function is realized by the axial movement of the
variable time group and the compensating group; the
repeater group consists of four lenses, the first lens is a double-convex
silicon positive lens, the second lens and the third lens are falcate
germanium negative lens facing to the image side, and the fourth lens is a
zinc selenide positive lens with the convex surface facing to the image side and is used for repeating an image and stabilizing an image surface in a zooming process. According to the optical
system, the double-component mechanical compensation zooming without the rear fixed group and the secondary
imaging technology are adopted, the 30 times of large zooming ratio is realized, the focal distance can be changed continuously in a range from 17.5mm to 525mm, the maximum
stroke of the zooming group is 106mm, the maximum
stroke of the compensating group is 50mm, the total optical length of the system is 350mm, and 100% of cold stop efficiency is realized.