Patent ID: 7236307

Claim:
A zoom optical system comprising, in order from an object side: a first lens unit with positive refracting power; a second lens unit with negative refracting power; a third lens unit with positive refracting power; and a fourth lens unit with positive refracting power, an aperture stop being interposed between the second lens unit and the fourth lens unit, at least, the first lens unit, the second lens unit, and the third lens unit being moved upon zooming so that spacings between individual lens units are varied, the first lens unit being composed of at most two lens elements, the second lens unit including, in order from the object side: a first negative lens in which an image-side surface is greater in curvature than an object-side surface; an air spacing; a second negative lens in which an image-side surface is greater in curvature than an object-side surface; an air spacing; and a positive lens in which an object-side surface is greater in curvature than an image-side surface, the second negative lens satisfying the following condition: −0.7< f 2/ R 23<0.05 where R 23 is a radius of curvature of the object-side surface of the second negative lens and f 2 is a focal length of the second lens unit, wherein the first lens unit is always moved toward the object side upon zooming from a wide-angle position to a telephoto position.