Zoom optical system

A zoom optical system comprising, arranged along an optical axis in order from the object side as follows:

• a first lens group having negative refractive power;
• a stop;
• and
• a second lens group having positive refractive power;
• wherein
• during zooming from the wide-angle end to the telephoto end, the first lens group moves along the optical axis toward the image side and the second lens group moves along the optical axis toward the object side;
• said second lens group includes, in order from the object side, an object-side lens element that has positive refractive power and at least one aspheric surface, and a biconvex lens element;
• wherein:
• the zoom optical system zooms from the wide-angle end to the telephoto end by moving said second lens group toward the object side along the optical axis in order to increase the focal length of the zoom optical system and compensates for movement of the image plane caused by this movement of said second lens group by moving said first lens group toward the image side as the zoom optical system zooms from the wide-angle end to the telephoto end of the zoom range;
• and
• the following conditions are satisfied:
• Nd21>1.55
• νd22>68
• 1.0<|f2/f1|<1.8
• where
• Nd21 is the index of refraction at the d-line of said object-side lens element,
• νd22 is the Abbe number at the d-line of said biconvex lens element,
• f2 is the focal length of said second lens group, and
• f1 is the focal length of said first lens group.