A zoom lens L0 consists of a first lens unit L1 having negative refractive power, a middle group Lm including one or more lens units, and a last lens unit having positive refractive power. The first lens unit L1 includes a first negative lens, a second negative lens, and a third negative lens that are arranged sequentially in an order from an object side to an image side. The number of negative lenses included in the first lens unit L1 is four or less. The middle group Lm includes a plurality of cemented lenses each having a cemented surface that is convex toward the object side, and includes a lens element Ln that is disposed closest to the image side among lens elements having negative refractive power included in the middle group Lm. The zoom lens L0 satisfies a predetermined inequality.

A zoom lens L0 consists of a first lens unit L1 having negative refractive power, a middle group Lm including one or more lens units, and a last lens unit having positive refractive power. The first lens unit L1 includes a first negative lens, a second negative lens, and a third negative lens that are arranged sequentially in an order from an object side to an image side. The number of negative lenses included in the first lens unit L1 is four or less. The middle group Lm includes a plurality of cemented lenses each having a cemented surface that is convex toward the object side, and includes a lens element Ln that is disposed closest to the image side among lens elements having negative refractive power included in the middle group Lm. The zoom lens L0 satisfies a predetermined inequality.

A lens system (10) for image pickup includes, in order from an object side (11), a first lens group (G1) that has negative refractive power and is fixed during focusing, a second lens group (G2) that has positive refractive power and moves during focusing, a third lens group (G3) that has positive refractive power and is fixed during focusing, and a fourth lens group (G4) that has a stop disposed on the object side, is disposed closest to an image plane side, has positive refractive power, and is fixed during focusing. The first lens group includes a first lens (L11) with positive refractive power that is disposed closest to the object side.

A lens system (10) for image pickup includes, in order from an object side (11), a first lens group (G1) that has negative refractive power and is fixed during focusing, a second lens group (G2) that has positive refractive power and moves during focusing, a third lens group (G3) that has positive refractive power and is fixed during focusing, and a fourth lens group (G4) that has a stop disposed on the object side, is disposed closest to an image plane side, has positive refractive power, and is fixed during focusing. The first lens group includes a first lens (L11) with positive refractive power that is disposed closest to the object side.

An optical imaging system includes six lens elements which are, in order from an object side to an image side along an optical path: 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. Each of the six lens elements has an object-side surface facing toward the object side and an image-side surface facing toward the image side. The first lens element has negative refractive power, and the object-side surface of the first lens element is concave in a paraxial region thereof. The image-side surface of the second lens element is concave in a paraxial region thereof. The fourth lens element has negative refractive power. The image-side surface of the sixth lens element is concave in a paraxial region thereof and has at least one critical point in an off-axis region thereof.

An optical imaging system includes six lens elements which are, in order from an object side to an image side along an optical path: 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. Each of the six lens elements has an object-side surface facing toward the object side and an image-side surface facing toward the image side. The first lens element has negative refractive power, and the object-side surface of the first lens element is concave in a paraxial region thereof. The image-side surface of the second lens element is concave in a paraxial region thereof. The fourth lens element has negative refractive power. The image-side surface of the sixth lens element is concave in a paraxial region thereof and has at least one critical point in an off-axis region thereof.

An optical imaging lens, in order from an object side to an image side along an optical axis, includes a first optical assembly, a second optical assembly, a third optical assembly, a fourth optical assembly, a fifth optical assembly, and a sixth optical assembly. The first optical assembly has positive refractive power. The fourth optical assembly has positive refractive power. The sixth optical assembly has negative refractive power. Two of the first optical assembly, the second optical assembly, the third optical assembly, the fourth optical assembly, the fifth optical assembly, and the sixth optical assembly include a compound lens with at least two lenses, while the others are a single lens, thereby providing a better optical performance of high image quality and low distortion.

An objective optical system includes in order from an object side, a front unit having a positive refractive power and a rear unit. The front unit includes in order from the object side, a first lens having a negative refractive power, a second lens having a positive refractive power, and a third lens having a positive refractive power. The rear unit includes one lens or a plurality of lenses and the following conditional expression is satisfied:
|(FLag−FLaC)/FLad|<0.05  (1) where, FLad denotes a focal length for a d-line of the front unit, FLag denotes a focal length for a g-line of the front unit, and FLaC denotes a focal length for a C-line of the front unit.

An objective optical system includes in order from an object side, a front unit having a positive refractive power and a rear unit. The front unit includes in order from the object side, a first lens having a negative refractive power, a second lens having a positive refractive power, and a third lens having a positive refractive power. The rear unit includes one lens or a plurality of lenses and the following conditional expression is satisfied:
|(FLag−FLaC)/FLad|<0.05  (1) where, FLad denotes a focal length for a d-line of the front unit, FLag denotes a focal length for a g-line of the front unit, and FLaC denotes a focal length for a C-line of the front unit.

An optical system for converting light from an object into parallel light, and guiding the parallel light to a plurality of image forming units each configured to form an image of the object includes a first lens unit having positive or negative refractive power and a second lens unit having positive refractive power that are disposed in order from an object side, wherein the first lens unit and the second lens unit are disposed at a widest interval in the optical system, and wherein the first lens unit consists of at least one positive lens and at least one negative lens that are disposed in order from the object side.