Patent ID: 7224535

Claim:
An infrared zoom lens system for forming a final image of an object, said system having an object side and an image side and forming a first intermediate real image between the object and the final image, said system comprising: a first lens group including at least two lens elements and located between the object and the first intermediate real image, said first lens group comprising at least one zoom subgroup which is moved to change the size (magnification) of the first intermediate real image; a second lens group including at least two lens elements and located between the first intermediate real image and the final image, at least a portion of which is moved to change the size (magnification) of the final image; and an optical stop located between the intermediate image and the final image; wherein off-axis field beam chief rays first cross an optical axis of the infrared zoom lens system on the object side of the intermediate image and then cross the optical axis again at the optical stop; and wherein a ratio of a maximum off-axis field beam rim ray to an axial field beam rim ray at a lens surface of the first lens group adjacent to object space at a longest focal length of the zoom lens system is less than about 1.25.