Patent ID: 8441556
Filing Date: 2013-05-14
Classification: G02B

Abstract:
1. An image forming optical system comprising, in order from its object side: a first lens unit having a positive refracting power; a second lens unit having a negative refracting power; a third lens unit having a positive refracting power; a fourth lens unit having a negative refracting power; and a fifth lens unit having a positive refracting power, wherein during zooming from the wide angle end to the telephoto end, the first lens unit, the second lens unit, the third lens unit, the fourth lens unit, and the fifth lens unit move, and the distances between the lens units change, the image forming optical system further comprises an aperture stop that moves integrally with the third lens unit during zooming, the first lens unit is located closer to the object side at the telephoto end than at the wide angle end, the third lens unit is located closer to the object side at the telephoto end than at the wide angle end, the first lens unit comprises, in order from the object side, a negative lens, a first positive lens, and a second positive lens, the second lens unit comprises a plurality of negative lenses and at least one positive lens, the third lens unit comprises a plurality of positive lenses and at least one negative lens, the fourth lens unit and the fifth lens unit each comprise two lenses or less, the refractive index of the at least one negative lens in the first lens unit for the d-line, the refractive index of the at least one positive lens in the second lens unit for the d-line, the refractive index of the at least one negative lens in the third lens unit for the d-line are not lower than 1.9, and the image forming optical system satisfies the following conditional expressions (1) and (2): where f 1 is the focal length of the first lens unit, f 2 is the focal length of the second lens unit, and f Tele is the focal length of the entire image forming optical system at the telephoto end.