An image plane illuminance distribution adjustable's - zoom lens

摘要

PURPOSE:To miniaturize a whole lens system by arranging at least one lens group which moves on an optical axis, also arranging an aperture mask which is variable in aperture between a lens closest to an image plane and the image plane, and varying the aperture of the aperture mask. CONSTITUTION:A zoom lens has a 1st group I for focusing which has positive refract ing power, a 2nd group II which has negative refracting power and moves toward the image plane at the time of variable power from the wide-angle end to the telephoto end, a 3rd group III which has fixed positive refracting power and also has the aperture stop in a lens system, a 4th group IV which has positive refracting power and moves toward the object side while correcting image plane variation caused by variable power at the time of the variable power from the wide-angle end to the telephoto end, and the aperture mask M which is arranged between the final lens and image plane S and fixed for the variable power. Thus the aperture stop ST is arranged in the 3rd group and the 2nd group and the 3rd group are moved respectively for the variable power to shorten the overall length of the lens and reduce the lens diame ter, and consequently the overall length of the lens system and lens diameter are reduced to miniaturize the whole lens system.