Focal length changeable lens system

摘要

The present invention provides a focal length changeable lens system which comprises a master lens group of a positive refractive power having at least one negative lens element made of a synthetic resin material; and a rear conversion lens group of a negative refractive power retractably arranged on the image side of the master lens group and having at least one positive lens element made of a synthetic resin material, in order to well compensate the change of back focal length of the lens system due to the change in temperature thereof without increasing the cost of the lens system.