There is provided an objective optical system, an image pickup apparatus, and an endoscope that are small in size and easy to assemble. The objective optical system includes, in order from the object side to the image side, a negative front lens group, an aperture stop, and a positive rear lens group. The front lens group includes a negative first lens disposed closest to the object, a meniscus second lens having a convex surface facing toward the image side, and a view-direction changing element. The objective optical system satisfies the following conditional expressions (1) to (4):

9<FL2/FL<15 (1)

0.3<FL1/FLf <0.64 (2)

95<(r2f+r2r)/(r2f-r2r)<−18 (3)

2.8<FLr/FL<4 (4).

A lens assembly includes a first lens, a second lens, a third lens, a fourth lens, a fifth lens, a sixth lens, and a seventh lens, all of which are arranged in order from an object side to an image side along an optical axis. The first lens is with negative refractive power. The second lens is a meniscus lens and includes a convex surface facing the object side and a concave surface facing the image side. The third lens includes a concave surface facing the object side. The fourth lens is with positive refractive power and includes a convex surface facing the object side. The fifth lens includes a convex surface facing the image side. The sixth lens is with negative refractive power. The seventh lens is with positive refractive power and includes a convex surface facing the object side. The fifth lens and the sixth lens are cemented.

A high-resolution imaging lens proofed against high-temperature instability includes a first lens with a negative power, a second lens with a negative power, a third lens with a positive power, a fourth lens with a negative power, a fifth lens with a positive power, and a sixth lens with a positive power. The first to the sixth lenses satisfy conditions of F1<0; 0.8>|F2/F6|>0.6, F2<0, F6>0; −3>F4/F5>−2, and 2.0<F/#. Each of the first lens, the third lens, the fourth lens, and the fifth lens is made of glass, each of the second lens and the sixth lens is made of plastic.

A lens system forms an image on an imaging element having a quadrilateral shape disposed on an optical axis. The lens system includes a second free-curved lens being asymmetrical with respect to the optical axis. A sag amount of the second free-curved lens in a circle separated from the optical axis by a length having a predetermined ratio with respect to a minimum image height has extrema outside of a first intersection points between a first surface passing through the optical axis and parallel to longer sides of the imaging element and the circle, and a second intersection point between a second surface passing through the optical axis and parallel to shorter sides of the imaging element and the circle. Each of the extrema is greater than the sag amount at the first intersection point or the second intersection point by 0.01 mm or greater.

A lens system is configured to form an image on an imaging element having a quadrilateral shape disposed on an optical axis, and includes a first free-curved lens being asymmetrical with respect to the optical axis. A free-curved surface of the first free-curved lens has positive refractive power at an intersection point between a circle separated from the optical axis by a length having a predetermined ratio with respect to a minimum image height and a first surface passing through the optical axis and parallel to longer sides of the imaging element, and negative refractive power at an intersection point between a circle separated from the optical axis by a length having the predetermined ratio with respect to the minimum image height and a second surface passing through the optical axis and parallel to shorter sides of the imaging element.

An image capturing lens assembly includes, in order from an object side to an image side, a first lens element, a second lens element, a third lens element, a fourth lens element, a fifth lens element, and a sixth lens element. The first lens element with negative refractive power has an image-side surface being concave in a paraxial region thereof. The second and third lens elements have refractive power. The fourth lens element has positive refractive power. The fifth lens element with negative refractive power has an object-side surface being concave in a paraxial region thereof. The sixth lens element with refractive power has an object-side surface being convex in a paraxial region thereof and an image-side surface being concave in a paraxial region thereof. The image capturing lens assembly has a total of six lens elements with refractive power.

An image capturing lens assembly includes, in order from an object side to an image side, a first lens element, a second lens element, a third lens element, a fourth lens element, a fifth lens element, and a sixth lens element. The first lens element with negative refractive power has an image-side surface being concave in a paraxial region thereof. The second and third lens elements have refractive power. The fourth lens element has positive refractive power. The fifth lens element with negative refractive power has an object-side surface being concave in a paraxial region thereof. The sixth lens element with refractive power has an object-side surface being convex in a paraxial region thereof and an image-side surface being concave in a paraxial region thereof. The image capturing lens assembly has a total of six lens elements with refractive power.

The present invention provides a camera lens consisting of eight lenses and having a small height, a wide angle, and good optical properties. The camera lens includes, sequentially from an object side, a first lens having a positive refractive power; a second lens having a positive refractive power; a third lens having a negative refractive power; a fourth lens having a positive refractive power; a fifth lens having a negative refractive power; a sixth lens having a positive refractive power; a seventh lens having a positive or negative refractive power; and an eighth lens having a negative refractive power. The camera lens satisfies specific conditions.

The present invention includes a camera optical lens including, from an object side to an image side in sequence: a first lens having a positive refractive power, a second lens having a negative refractive power, a third lens having a refractive power, a fourth lens having a positive refractive power, and a fifth lens having a negative refractive power. The camera optical lens satisfies the following conditions: 0.18≤d1/TTL≤0.40, 0.60≤ET1/d1≤1.00, −7.00≤f2/f≤−2.00, and 3.00≤R6/R5≤12.00. The camera optical lens according to the present invention has excellent optical characteristics, such as large aperture, wide angle, and ultra-thin.

The endoscope objective optical system includes in order from an object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power. A lens surface positioned nearest to an image side in the second lens group is a concave surface which is directed toward the image side. The second lens group moves along an optical axis.