From infrared ray zoom lens and infrared ray

摘要

A small-sized and low-cost infrared zoom lens while maintaining the brightness of an image and relevant arts to the infrared zoom lens is provided. The infrared zoom lens 1a is made up of first to third lens groups G1 to G3 formed of zinc sulfide. The first lens group G1 is made up of one or two lenses and has a positive refractive power. The second lens group G2 is made up of one or two lenses and has a negative refractive power. The third lens group G3 is made up of two or more lenses and has a positive refractive power as the whole lens group and also includes a positive meniscus lens with a convex face pointed at the object side as a final lens on the image surface side. At the zooming time, the second lens group G2 is moved along the optical axis. At least one of the lens surfaces of the first to third lens groups G1 to G3 is a diffraction surface. At least one of the lens surfaces of the first and third lens groups G1 and G3 is an aspheric surface.