Abstract: A zoom optical system comprising, in order from the object side, a first lens unit with negative power, a second lens unit with positive power, a third lens unit with positive power, and a fourth lens unit, wherein spacing between the first lens unit and the fourth lens unit is fixed to an image surface, and the second lens unit and the third lens unit are moved on an optical axis so as to change spacings between the respective lens units when the magnification of the zoom optical system is changed. The third lens unit comprises lens elements more than three, including an aperture stop, at least a lens element with positive power and at least a lens element with negative power, and the most image-side lens element of the third lens unit is a meniscus lens element with a concave surface facing the image side.

Abstract: An optical system has a positive lens group, a negative lens group, and an aperture stop. The negative lens group is disposed at an object side of the aperture stop and has a cemented lens, and in a rectangular coordinate system in which a horizontal axis is Nd and a vertical axis is ?d, when a straight line indicated by Nd=?×?d+? where ?=?0.017 is set, Nd and ?d of at least one lens forming the cemented lens are included in both an area determined by a line when a lower limit value is in a range of expression (1a) and a line when an upper limit value is in a range of the expression (1a), and an area determined by expressions (2a) and (3a): 1.45<?<2.15 (1a); 1.30<Nd<2.20 (2a); and 3<?d<12 (3a); where Nd denotes a refractive index, and ?d denotes an Abbe's number.

Abstract: An image forming optical system of the present invention includes at least one cemented lens which includes a first lens element (e1), a second lens element (e2), and a third lens element (e3). The first lens element e1 is cemented to a surface on one side of the second lens element e2, and the third lens element e3 is cemented to the other surface of the second lens element e2. The first lens element e1 is a positive lens, and a combined refracting power of the second lens element e2 and the third lens element e3 is negative. The cemented lens satisfies a predetermined conditional expression.

Abstract: An image forming optical system characterized by comprising a positive first lens group, a negative second lens group, and third and subsequent lens groups having a positive composite refracting power, wherein the first lens group is composed of a cemented lens made up of one positive lens and one negative lens arranged in order from the object side, and the image forming optical system satisfies the following conditional expressions (1-1) and (2-1): 0.05<T1g/Flt<0.10??(1-1) 0.50<?gF<0.75??(2-1) where T1g is the thickness of the first lens group on the optical axis, Flt is the focal length of the entire image forming optical system at the telephoto end, and ?gF is the partial dispersion ratio (ng1?nF1)/(nF1?nC1) of the negative lens, where nd1, nC1, nF1, and ng1 are refractive indices of the negative lens respectively for the d-line, C-line, F-line, and g-line.

Abstract: In an image forming optical system having a positive lens group, a negative lens group, and an aperture stop, the negative lens group is disposed at an image side of the of the aperture stop, the negative lens group has a cemented lens which is formed by cementing a plurality of lenses, and in a rectangular coordinate system in which, a horizontal axis is let to be Nd and a vertical axis is let to be ?d, when a straight line indicated by Nd=?×?d+? (where, ?=?0.017) is set, Nd and ?d of at least one lens forming the cemented lens are included in both of areas namely, an area which is determined by a line when a lower limit value is in a range of a following conditional expression (1), and a line when an upper limit value is in a range of the following conditional expression (1), and an area determined by following conditional expressions (2) and (3). 1.45<?<2.15??(1) 1.30<Nd<2.20??(2) 3<?d<12??(3) Here, Nd denotes a refractive index, and ?d denotes an Abbe's number.

Abstract: In an image forming optical system having a positive lens group, a negative lens group, and an aperture stop, the negative lens group is disposed at an image side of the of the aperture stop, the negative lens group has a cemented lens which is formed by cementing a plurality of lenses, and in a rectangular coordinate system in which, a horizontal axis is let to be Nd and a vertical axis is let to be ?d, when a straight line indicated by Nd=?×?d+? (where, ?=?0.017) is set, Nd and ?d of at least one lens forming the cemented lens are included in both of areas namely, an area which is determined by a line when a lower limit value is in a range of a following conditional expression (1), and a line when an upper limit value is in a range of the following conditional expression (1), and an area determined by following conditional expressions (2) and (3). 1.45<??2.15??(1) 1.30<Nd<2.20??(2) 3<?d<12??(3) Here, Nd denotes a refractive index, and ?d denotes an Abbe's number.

Abstract: An image forming optical system according to the present invention is characterized in that, in an image forming optical system having a positive lens group, a negative lens group, and an aperture stop, the positive lens group is disposed at an image-plane side of the aperture stop, the positive lens group has a cemented lens which is formed by cementing a plurality of lenses, in a rectangular coordinate system in which, a horizontal axis is let to be Nd and a vertical axis is let to be ?d, when a straight line indicated by Nd=?×?d+? (where, ?=?0.017) is set, Nd and ?d of at least one lens forming the cemented lens are included in both of areas namely, an area which is determined by a line when a lower limit value is in a range of a following conditional expression (1), and a line when an upper limit value is in a range of the following conditional expression (1), and in an area determined by following conditional expressions (2) and (3) 1.45<??2.15??(1) 1.30<Nd<2.

Abstract: An image forming optical system according to the present invention is characterized in that, in an image forming optical system having a positive lens group, a negative lens group, and an aperture stop, the negative lens group is disposed at an object side of the aperture stop, the negative lens group has a cemented lens which is formed by cementing a plurality of lenses, and in a rectangular coordinate system in which, a horizontal axis is let to be Nd and a vertical axis is let to be ?d, when a straight line indicated by Nd=?×?d+? (where, ?=?0.017) is set, Nd and ?d of at least one lens forming the cemented lens are included in both of areas namely, an area which is determined by a line when a lower limit value is in a range of a following conditional expression (1a), and a line when an upper limit value is in a range of the following conditional expression (1a), and an area determined by following conditional expressions (2a) and (3a). 1.45<?<2.15??(1a) 1.30<Nd<2.

Abstract: An image forming optical system according to the present invention is characterized in that, in an image forming optical system having a positive lens group, a negative lens group, and an aperture stop, the positive lens group is disposed at an object side of the aperture stop, the positive lens group has a cemented lens which is formed by cementing a plurality of lenses, in a rectangular coordinate system in which a horizontal axis is let to be Nd and a vertical axis is let to be ?d, when a straight line indicated by Nd=?×?d+? (where, ?=?0.017) is set, Nd and ?d of at least one lens forming the cemented lens is included in both of areas namely, an area which is determined by a line when a lower limit value is in a range of a following conditional expression (1a), and a line when an upper limit value is in a range of the following conditional expression (1a), and of an area determined by following conditional expressions (2a) and (3a). 1.45<?<2.15??(1a) 1.30<Nd<2.