A projection lens including a first lens group, an aperture stop, a second lens group, and a reflective optical element sequentially arranged from a reduction side to a magnification side along an optical axis is provided. The first lens group has positive refractive power. The second lens group has negative refractive power. The reflective optical element has positive refractive power. The projection lens satisfies 1.2<|F.sub.2/F.sub.1|<3.5, wherein F.sub.2 is an effective focal length of the reflective optical element, and F.sub.1 is an effective focal length of the first lens group and the second lens group. A projection apparatus is also provided.

A projection lens including a first lens group, an aperture stop, a second lens group, and a reflective optical element sequentially arranged from a reduction side to a magnification side along an optical axis is provided. The first lens group has positive refractive power. The second lens group has negative refractive power. The reflective optical element has positive refractive power. The projection lens satisfies 1.2<|F.sub.2/F.sub.1|<3.5, wherein F.sub.2 is an effective focal length of the reflective optical element, and F.sub.1 is an effective focal length of the first lens group and the second lens group. A projection apparatus is also provided.

An optical system includes a first lens group including at least one lens, and a second lens group including at least one lens, wherein the first lens group and the second lens group are disposed in order from an object side, wherein each of the first lens group and the second lens group is configured to move in an optical axis direction to change a magnification between a wide-angle end and a telephoto end, and wherein G1F is a focal length of the first lens group, G2F is a focal length of the second lens group, and |G1F/G2F| has a value of between 0.5 and 1.

An optical system includes a first lens group including at least one lens, and a second lens group including at least one lens, wherein the first lens group and the second lens group are disposed in order from an object side, wherein each of the first lens group and the second lens group is configured to move in an optical axis direction to change a magnification between a wide-angle end and a telephoto end, and wherein G1F is a focal length of the first lens group, G2F is a focal length of the second lens group, and |G1F/G2F| has a value of between 0.5 and 1.

Provided is an image-capturing optical system including a first lens group that includes a first lens having a positive refractive power, a second lens having a negative refractive power, and a third lens having a positive refractive power arranged in this order starting from an object side or that includes a first lens having a negative refractive power and a second lens having a positive refractive power arranged in this order starting from the object side, the first lens group having a negative refractive power as a whole, at least one on-axis luminous flux regulating diaphragm, and a second lens group that has a positive refractive power. A surface of at least one of the lens that are included in the first lens group and that have the negative refractive power, the surface being located on the object side, is a convex surface in a paraxial region and has an aspherical surface having a shape with which convex power decreases with increasing distance from an optical axis. A surface of the second lens group on the most image side has a convex shape toward the image side in a paraxial region. The image-capturing optical system satisfies the following conditional expressions (1) to (3):

−1.30<fn/f<−0.6   (1)

Di/tan ω/100<−0.4   (2)

2ω≤120°  (3)

where f is a focal length of an entire lens system, fn is a focal length of a negative lens of the first lens group, Di is a distortion at a maximum angle of view (unit: %), and ω: an incident angle of a maximum angle of view light beam on the object side.

Provided is an image-capturing optical system including a first lens group that includes a first lens having a positive refractive power, a second lens having a negative refractive power, and a third lens having a positive refractive power arranged in this order starting from an object side or that includes a first lens having a negative refractive power and a second lens having a positive refractive power arranged in this order starting from the object side, the first lens group having a negative refractive power as a whole, at least one on-axis luminous flux regulating diaphragm, and a second lens group that has a positive refractive power. A surface of at least one of the lens that are included in the first lens group and that have the negative refractive power, the surface being located on the object side, is a convex surface in a paraxial region and has an aspherical surface having a shape with which convex power decreases with increasing distance from an optical axis. A surface of the second lens group on the most image side has a convex shape toward the image side in a paraxial region. The image-capturing optical system satisfies the following conditional expressions (1) to (3):

−1.30<fn/f<−0.6   (1)

Di/tan ω/100<−0.4   (2)

2ω≤120°  (3)

where f is a focal length of an entire lens system, fn is a focal length of a negative lens of the first lens group, Di is a distortion at a maximum angle of view (unit: %), and ω: an incident angle of a maximum angle of view light beam on the object side.

An imaging lens system includes an optical path folding member including a forwardmost reflective surface disposed closest to an object side, a rearmost reflective surface disposed closest to an imaging plane, and a rear reflective surface disposed to form an acute angle with the rearmost reflective surface and configured to reflect light reflected by the rearmost reflective surface to the imaging plane, and a first lens group disposed on an object side of the forwardmost reflective surface or an image side of the forwardmost reflective surface, wherein an angle between a first virtual plane including the forwardmost reflective surface and a second virtual plane including the rearmost reflective surface is 15 to 27 degrees.

An imaging lens system includes an optical path folding member including a forwardmost reflective surface disposed closest to an object side, a rearmost reflective surface disposed closest to an imaging plane, and a rear reflective surface disposed to form an acute angle with the rearmost reflective surface and configured to reflect light reflected by the rearmost reflective surface to the imaging plane, and a first lens group disposed on an object side of the forwardmost reflective surface or an image side of the forwardmost reflective surface, wherein an angle between a first virtual plane including the forwardmost reflective surface and a second virtual plane including the rearmost reflective surface is 15 to 27 degrees.

A microscope objective lens (OL) includes a first lens group (G1) and a second lens group (G2) arranged in order from the object side. The first lens group (G1) has positive refractive power and converts luminous flux from an object into convergent luminous flux. The second lens group (G2) has negative refractive power and converts the convergent luminous flux from the first lens group (G1) into parallel luminous flux. The microscope objective lens (OL) satisfies the following conditional expression: 14.0≤NA×f 1.0<H1/HO Where NA is the numerical aperture of the microscope objective lens (OL); f is the focal distance of the microscope objective lens (OL); H1 is the maximum value of the height of marginal rays from an on-axis object point of the first lens group (G1); and HO is the height of marginal rays on the lens surface closest to the image side of the second lens group (G2).

A microscope objective lens (OL) includes a first lens group (G1) and a second lens group (G2) arranged in order from the object side. The first lens group (G1) has positive refractive power and converts luminous flux from an object into convergent luminous flux. The second lens group (G2) has negative refractive power and converts the convergent luminous flux from the first lens group (G1) into parallel luminous flux. The microscope objective lens (OL) satisfies the following conditional expression: 14.0≤NA×f 1.0<H1/HO Where NA is the numerical aperture of the microscope objective lens (OL); f is the focal distance of the microscope objective lens (OL); H1 is the maximum value of the height of marginal rays from an on-axis object point of the first lens group (G1); and HO is the height of marginal rays on the lens surface closest to the image side of the second lens group (G2).