Image forming optical system and electronic image pickup apparatus using the same

An image forming optical system comprising, in order from the object side to the image side, a first lens group having a positive refracting power, a second lens group having a negative refracting power, and an image side lens group having a positive refracting power, wherein the distance between the first lens group and the second lens group changes during zooming, wherein

• a refractive optical element A comprising a positive refracting power is provided in the first lens group, and
• the image forming optical system satisfies the following conditional expressions (4-1), (4-2), and (2):
• νdA<30 (4-1),
• 0.54<θgFA<0.92 (4-2), and
• |fG1/fG2|>6.4 (2),
• where ndA, nCA, nFA, and ngA are the refractive indices of the refractive optical element A for the d-line, the C-line, the F-line, and the g-line respectively, νdA is the Abbe constant (ndA−1)/(nFA−nCA) of the refractive optical element A, θgFA is the relative partial dispersion (ngA−nFA)/(nFA−nCA) of the refractive optical element A, fG1 is the focal length of the first lens group, and fG2 is the focal length of the second lens group.

An image forming optical system comprising, in order from the object side to the image side, a first lens group having a positive refracting power, a second lens group having a negative refracting power, and an image side lens group having a positive refracting power, wherein the distance between the first lens group and the second lens group changes during zooming, wherein

• a refractive optical element A comprising a positive refracting power is provided in the first lens group,
• the refractive optical element A is located closest to the object side in the first lens group, and
• the image forming optical system satisfies the following conditional expressions (4-1), (4-2), and (7):
• νdA<30 (4-1),
• 0.54<θgFA<0.92 (4-2), and
• 0.8<fA/fG1<13.0 (7),
• where ndA, nCA, nFA, and ngA are the refractive indices of the refractive optical element A for the d-line, the C-line, the F-line, and the g-line respectively, νdA is the Abbe constant (ndA−1)/(nFA−nCA) of the refractive optical element A, θgFA is the relative partial dispersion (ngA−nFA)/(nFA−nCA) of the refractive optical element A, fA is the focal length of the refractive optical element A, and fG1 is the focal length of the first lens group.

An electronic image pickup apparatus comprising an image forming optical system and an image pickup element, wherein the image forming optical system comprises, in order from the object side to the image side, a first lens group having a positive refracting power, a second lens group having a negative refracting power, and an image side lens group having a positive refracting power, the distance between the first lens group and the second lens group changes during zooming, a refractive optical element A having a positive refracting power is provided in the first lens group, and the refractive optical element A satisfies the following conditional expression (3-2):

• 0<(Zb(3.3a)−Za(3.3a))/(Zb(2.5a)−Za(2.5a))<0.990 (3-2),
• where fw is the focal length of the image forming optical system at the wide angle end, ft is the focal length of the image forming optical system at the telephoto end, IH is the largest image height on the image pickup element, Za(h) is the distance along the optical axis between the object side surface vertex of the refractive optical element A on the optical axis and a point on the object side surface of the refractive optical element A at height h, Zb(h) is the distance along the optical axis between the object side surface vertex of the refractive optical element A on the optical axis and a point on the image plane side surface of the refractive optical element A at height h, and a is a value defined by the following equation (3-1):
• a={(IH)2×log10(ft/fw)}/fw (3-1).