Abstract: An infrared zoom lens having no aspheric lens elements comprises, six distinct lens groups each having two or less spherical lens elements. The first group has a positive power, the second group has a negative power, the third group has a positive power, the fourth group has a negative power, the fifth has a positive power and the sixth group has either a positive or negative power with the purpose of relaying and focusing the output of the fourth lens group onto the focal plane of the imaging device. The second and fourth groups are axially movable but fixed relative to each other and move as a group relative to stationary groups one and three. The zoom ratio is thus varied, and no additional motion is required to keep the imagery sharp at the output focal plane as the lens is varied from the wide angle to the telephoto position.